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The beds which support Geological Basins frequently thin towards the centres of those basins, thus producing, wholly or in part, the basined form of the strata. It was, however, shown that the beds of the basins themselves frequently thicken towards the centre of the basins. February 25th.—John Evans, Esq., F.R.S., President, in the Chair.
The following communications were read :
1. “ Geological Notes on a Journey from Algiers to the Sahara.” By George Maw, Esq., F.L.S., F.G.S.
The author commences by describing the details observed on his journey from Algiers to L’Aghouat, on the borders of the Sahara. The distance traversed was 285 miles, or about 210 miles in a straight line, and in a direction nearly north and south. No eruptive rocks were observed. The oldest rock is a boss of mica-schist and gneiss behind the city of Algiers; it forms a low anticlinal, with a N. and S. strike. The pass through the gorge of the Chiffa in the Lesser Atlas shows hard slaty rocks dipping S. at a high angle; they are repeated as an anticlinal on the south side of the higher part of the Tell plateau, and are probably Mesozoic. In the plain separating the Tell from the Hauts Plateaux, and on the south side of the latter, red and yellow sandstones form anticlinals; these rocks resemble the Bunter in mineral characters, and are overlain by red marls resembling the Keuper. In the northern escarpment of the Hauts Plateaux saliferous marls are exposed, interstratified between the sandstones and below the red and grey marls. Crystals of salt and gypsum are intimately mixed with the grey marls; and the so-called “ Rochers de Sel” are capped with great blocks of rock tumbled about in confusion, the position of which the author ascribes to the failure of support due to the solution of the salt in the underlying salt-marls. A thin series of bright red and green marls is seen to overlie the Red Sandstones in several places; and above this is an immense series of dark grey marls, interstratified with argillaceo-calcareous bands, forming a great synclinal of the Hauts Plateaux, and a contorted mass on the Tell plateau. These are probably cretaceous. At L’Aghouat they are overlain by fossiliferous beds, probably of Miocene age. Other Tertiary beds observed are soft yellow calcareous freestones on the flanks of the promontory of Algiers and of the Lesser Atlas, and some red and grey marls and ferruginous freestone capping the Tell plateau, the former at a height of 100-900 feet, and the latter of 2500-4000 feet above the sealevel. The plain of the Mitidja, between the Lesser Atlas and Algiers, consists of grey loam with shingle-beds, of post-tertiary age. A similar loam covers the great plain of the northern Sahara, and rises to a height of 2700 feet. Raised beaches occur on the coast up to an elevation of 600 feet above the sea-level; and similar beaches are found inland, south of the Tell plateau, at a height of 2000 feet.
The oldest land in the line of section is the anticlinal of mica schist near Algiers, the strike of which is nearly at right angles to that of the other rocks. The upheaval of the Mesozoic rocks was contemporaneous with the first upheaval of the Lesser Atlas ; it was followed by a long period of denudation, and this by a subsidence of at least 3000 feet in Tertiary times, during which the Miocene deposits were formed. The Tell plateau was thus elevated at least 4000 feet, and the district north of the Lesser Atlas at least 1000 feet, the north face of those mountains probably marking a posttertiary line of fault of 3000 feet. This operation was followed by a long period of denudation, and this by a post-tertiary depression, which the author terms the “ Sahara submergence,” after which the land was reelevated at least 3000 feet, but perhaps considerably
A gradual subsidence appears to be still taking place.
2. “On the Trimerellidæ, a Palæozoic Family of the Palliobranchs or Brachiopoda." By Thomas Davidson, Esq., F.R.S., F.G.S., &c., and Prof. William King, Sc.D.
In this memoir the authors describe in detail certain Brachiopoda, for which they propose to establish a distinct family, discuss the characters and affinities of the family, and indicate certain geological considerations which arise from their study of its members. The first known species were described in 1853 under the names of Obolus Davidsoni and 0. transversus ; but in 1862 Mr. Billings described an allied form as constituting a new genus called Trimerella. With this Gotlandia of Dall (1870) is identical. In 1871 Prof. Hall proposed the new genera Rhymobolus and Dinobolus ; and in the same year Mr. Billings established the genus
Monomerella. Rhynobolus is regarded by the authors as a synonym of Trimerella ; and several other proposed genera being similarly cancelled, the authors retain only the genera Trimerella, Dinobolus, and Monomerella as constituting their family Trimerellidæ.
The Trimerellidæ are described as having a calcareous shell, generally massive, with the umbo of the pedicle-valve often large, pointed, solid or hollow; area usually of considerable size, with a large solid deltidium ; hinge of both valves slightly dentary ; in the interior of each valve there is a more or less elevated platform, longitudinally placed, and either solid or doubly vaulted ; and from the
middle of its anterior end a median plate occasionally projects into the anterior half of the valve, especially in the brachial valve.
In discussing the affinities of the new family, the authors enter in considerable detail into the arrangement and functions of the muscles of the Brachiopoda, and show, from the muscular indications and other characters of the Trimerellidæ, that they are most nearly allied to the Lingulidæ, although presenting characters of sufficient importance to warrant their separation as a distinct family.
As far as is at present known, the Trimerellidæ are confined to the Cambro-Silurian and Silurian systems; but the authors indicate the probability that in the Cambrian or early Cambro-Silurian rocks a generalized form will hereafter be detected, bringing Discina, Lingula, and Obolus into close relationship, as regards their myology, with each other and with Dinobolus. Such a form would constitute the root from which the Trimerellidæ have originated. The
genera and species described are as follows :: Genus 1. Trimerella, Billings (1862). Sp. 1. T. grandis, Bill. ; 2. T. acuminata, Bill. ; 3. T. Lindströmi, Dall; 4. T. Billingsü, Dall; 5. T. ? galtensis, Bill., = minor, Dall; 6. T. ohioensis, Meek ; 7. T. Dalli, Dav. & King; 8. T. wisbyensis, Dav. & King.
Genus 2. Monomerella, Billings (1871). Sp. 1. M. prisca, Bill. : 2. M. Walmstedti, Dav. & King; 3. M. Lindströmi, Dav. & King; 4. M. orbicularis, Bill.
Genus 3. Dinobolus, Hall (1871). Sp. 1. D. Conradi, Hall; 2. D. Davidsoni, Salt.; 3. D. canadensis, Bill. ; 4. D. transversus, Salt. ; 5. D. Woodwardi, Salt. ; 6. D. magnificus, Bill. ; 7. D. Schmidti, sp. n.
The authors further discuss the characters of Professor Hall's Lingulops Whitfieldi, which they regard as in some respects intermediate between the Lingulidæ and Trimerellidæ, and describe under the name of Chelodes Bergmani a fossil sent to them by Dr. Lindström under the impression that it was a Trimerellid. They indicate its points of resemblance to Trimerella, but think that it really belongs to the section of the Calenterata represented by Calceola and Goniophyllum. The fossil is from the isle of Gothland, and was obtained from a formation equivalent to the Aymestry limestone.
X. Intelligence and Miscellaneous Articles. PERMANENT ICE IN A MINE IN THE ROCKY MOUNTAINS.
BY R. WEISER, OF GEORGETOWN, COLORADO.
rocks found in some of our silver-mines in Clear Creek Co., Colorado. I will first give a statement of the facts in the case, and then a theory for their explanation.
There is a silver-mine high up on McClellan Mountain, called the “ Stevens Mine.” The altitude of this mine is 12,500 feet. At the depth of from 60 to 200 feet the crevice matter, consisting of silica, calcite, and ore, together with the surrounding wall-rocks, is found to be in a solid frozen mass. McClellan Mountain is one of the bighest eastern spurs of the Snowy Range; it has the form of a horseshoe, with a bold escarpment of felspathic rock near 2000 feet high, which in some places is nearly perpendicular. The Stevens Mine is situated in the south-western bed of the great horseshoe ; it opens from the north-western. A tunnel is driven into the mountain on the lode, where the rock is almost perpendicular. Nothing unusual occurred until a distance of some 80 or 90 feet was made; and then the frozen territory was reached, and it has continued for over 200 feet. There are no indications of a thaw summer or winter ; the whole frozen territory is surrounded by hard massive rock, and the lode itself is as hard and solid as the rock. The miners being unable to excavate the frozen material by pick or drill to get out the ore (for it is a rich lode, running argentiferous galena from 5 to 1200 ounces to the ton), found the only way was to kindle a large wood fire at night against the back end of the tunnel and thus thaw the frozen material, and in the morning take out the disintegrated ore. This has been the mode of mining for more than two years. The tunnel is over 200 feet deep, and there is no diminution of the frost ; it seems to be rather increasing. There is, so far as we can see, no opening or channel through which the frost could possibly have reached such a depth
from the surface. There are other mines in the same vicinity in a like frozen state.
From what we know of the depth to which frost usually penetrates into the earth, it does not appear probable that it could have reached the depth of 200 feet through the solid rock in the Stevens Mine, nor even through the crevice matter of the lode, which, as we have stated, is as hard as the rock itself. The idea, then, of the frost reaching such a depth from the outside being utterly untenable, I can see no other way than to fall back upon the Glacial era of the Quaternary. Evidences of the Glacial Period are found all over the Rocky Mountains. Just above the Stevens Mine there are the remains of a moraine nearly a mile long, and half a mile wide. The débris of this moraine consists of small square and angular stones, clearly showing that they have not come from any great distance. And just over the range, on the Pacific slope, there are the remains of the largest moraine I have ever seen, consisting of felspathic boulders of immense size. I conclude, therefore, that it was during that period of intense cold that the frost penetrated so far down into these rocks, and that it has been there ever since, and bids fair to remain for a long time to come.—Silliman's American Journal, December 1874.
ON THE MIXTURE OF COLOURS IN BINOCULAR VISION.
BY W. VON BEZOLD*. The Author has succeeded in clearing up and reconciling the divergent observations of different observers on the mixed colours produced when the two eyes receive light of different colours. Some, as Dove or Ludwig, assert most positively that it is possible to obtain a combined colour by the binocular fusion of two images of different colours; while Franke, Helmholtz, and others advance precisely the contrary.
According to the Author, the combined tint is obtained when, with one and the same accommodation, the differently coloured surfaces are both at the distance of distinct vision. When this is not the case, there is a struggle between the two visual fields, and one or the other prevails.-Bibliothèque Universelle, Archives des Sciences, No 202, p. 184. EXPERIMENTAL STUDIES ON THE ELECTRICAL RESISTANCE OF
METALS AND ITS VARIATION UNDER THE INFLUENCE OF TEMPERATURE. BY RENÉ BENOITT.
Resuming the important question of the electric conductivity of metals, M. Benoit first measured very accurately the specific resistance of a number of them at 0° C. For this he operated comparatively with two different methods—that of the differential galvanometer, and that of the Wheatstone bridge ; and he made numerous determinations on various specimens of each metal. The following Table contains the means of all the results, which are referred to the two units now most generally adopted for the measurement of * Pogg. Ann. Jubelband, p.
From the surface. There are other mines in the same vicinity in a ke frozen state.
From what we know of the depth to which frost usually peneates into the earth, it does not appear probable that it could have eached the depth of 200 feet through the solid rock in the tevens Mine, nor even through the crevice matter of the lode, hich, as we have stated, is as hard as the rock itself. The idea, men, of the frost reaching such a depth from the outside being tterly untenable, I can see no other way than to fall back upon the lacial era of the Quaternary. Evidences of the Glacial Period re found all over the Rocky Mountains. Just above the Stevens Tine there are the remains of a moraine nearly a mile long, and alf a mile wide. The débris of this moraine consists of small quare and angular stones, clearly showing that they have not come -om any great distance. And just over the range, on the Pacific ope, there are the remains of the largest moraine I have ever een, consisting of felspathic boulders of immense size. I conude, therefore, that it was during that period of intense cold that me frost penetrated so far down into these rocks, and that it has een there ever since, and bids fair to remain for a long time to ome.-Silliman's American Journal, December 1874.
ON THE MIXTURE OF COLOURS IN BINOCULAR VISION.
The Author has succeeded in clearing up and reconciling the ivergent observations of different observers on the mixed colours roduced when the two eyes receive light of different colours. ome, as Dove or Ludwig, assert most positively that it is possible obtain a combined colour by the binocular fusion of two images different colours; while Franke, Helmholtz, and others advance ecisely the contrary.
According to the Author, the combined tint is obtained when,
one and the same accommodation, the differently coloured sures are both at the distance of distinct vision. When this is not
case, there is a struggle between the two visual fields, and one he other prevails.-Bibliothèque Universelle, Archives des Sciences, 202, p. 184.
ERIMENTAL STUDIES ON THE ELECTRICAL RESISTANCE OF
esuming the important question of the electric conductivity of ls, M. Benoit first measured very accurately the specific resist of a number of them at 0° C. For this he operated comparawith two different methods-that of the differential galvanor, and that of the Wheatstone bridge; and he made numerous minations on various specimens of each metal. The following contains the means of all the results, which are referred to
vo units now most generally adopted for the measurement of
to the Paris Faculty of Sciences.
*Pogg. Ann. Jubelband, p. 585.
† A thesis presented
conductivities, viz. the British-Association unit, or ohm, and the Siemens unit.
Pure silver, annealed..
Aluminium bronze, annealed†
German silver, annealed ‡..
From the third column, which gives the specific conductivities of these metals referred to that of silver, it is seen that the results ohtained by the author differ but little from those which have been given by other experimenters.
The diminution undergone by the electric conductivity of the metals with rise of temperature was the essential aim of M. Benoit's researches. This influence of temperature, already remarked by Davy, was studied by Ed. Becquerel, who measured it up to 100°, by Matthiessen, who followed it as far as 200°, and by other physicists. M. Benoit has set himself the task of studying this variation of conductivity within the most extended limits of temperature. The conducting wire on which he operated was wrapped spirally round a cylindrical support of pipeclay, enclosed in a muffle which was immersed in a bath of a volatile liquid heated by aid of a Perrot furnace. The liquid was water, mercury, sulphur, or cadmium, by means of which constant temperatures were obtained of 100°, 360°, 440°, and 860°. Moreover a great number of experiments were made above 360° by aid of a mercury bath, the temperature of which was regulated. These measurements were corrected for the dilatation.
Plates annexed to the memoir give the graphic representation of the results obtained. They show that the increase of the resistance follows a regular course which continues, probably, for all the metals, as for tin, lead, and zinc, as far as their melting-point §. *From the wiredrawers: copper 64.2, zinc 33·1, lead 0'4, tin 0'4. † Copper 90, aluminium 10. Copper 50, nickel 25, zinc 25. § Fusion is in general accompanied, as we know from the researches of