NOTE BY MR. VAN DER POL, JUNR. The well-known facts respecting the division of current between a telephone and its shunt circuit are correctly stated by Prof. Howe, but he has rather lost sight of the motive of my paper and of certain experimental difficulties. The principal object in view was to test whether the audibility factor could be considered to vary as suggested by Prof. Love with the received antenna-current. In discussing some experiments by Dr. Austin, Prof. Love makes use of the audibility factor defined as R+S/S, where S is the resistance of the shunt and R the telephone resistance*. In order to test experimentally his suggestions as to the proportionality of the so-defined audibility factor to the first power or square of the antenna-current I had to use the same constants. My experimental results appeared to be in close agreement with Prof. Love's suggestions. The same definition (with the aid of the telephone resistance) of the audibility factor is used by several other writers. Prof. Howe refers in his paper to a very recent publication by Austin which was published after my paper had been sent to the Philosophical Magazine. Here Austin refers to the telephone-impedance, but on the other hand, in a former paper by the same experimentalist, he defines the audibility factor using the telephone resistance instead of the impedance t. It is by no means clear whether Austin or Hogan employed the true impedance of their telephones in the audibility factor, as in their papers cited no references at all are given how they determined these impedances. Further, it is a matter of considerable difficulty to measure the true impedance of a telephone when used as in Wireless Telegraphy in series with a crystal detector, and therefore traversed by an intermittent or pulsatory current, the waveform of which is not known. From the pronounced variation in character of the tone in the telephone-receiver with different couplings it may further be concluded that, probably as a consequence of the irregular shape of the characteristic of most crystal detectors, the telephone current, while varying in intensity also (opposite to the suggestion of Prof. Howe) varies in wave-form, so that it is doubtful if the ordinary well-known theory of sine-form currents may be applied to the shunted telephone method. Moreover, the current in the telephone circuit at the † Bull. Bureau of Standards, vi. no. 4, p. 531 (1910). See also J. Erskine Murray, 'A Handbook of Wireless Telegraphy' (1914), p. 349. moment when the measurement is made is extremely small and quite beyond reach of any thermo-electric ammeter. The writer is therefore of opinion that an exact experimental determination of the telephone impedance under actual working conditions is a matter of higher order of difficulty than the measurement of received antenna-current itself. It must further be borne in mind that in any case the shunt value which quenches the telephone sound is difficult to determine in practice with any but a rough approximation. In a very quiet room it may perhaps be determined to within 5 or 10 per cent., but in a wireless station or on board ship perhaps not within 30 or 40 per cent. No assumptions as that made by Prof. Howe that the true impedance of the telephone under actual working conditions is equal to four times the steady resistance has been justified by any experiments. Hence, to avoid suppositions not based on experiment, the value taken for the calculation of the audibility factor in the case of my experiments was the steady resistance, although I was perfectly well aware that this was not identical with the true impedance for the waveform and frequencies used. Having regard to the uncertainty attending the constants employed by Austin and Hogan, and the difficulty of determining exact values, it seemed better to base the reduction of the observations on known measurements rather than on assumptions as to the ratio of impedance to resistance. XIV. Proceedings of Learned Societies. GEOLOGICAL SOCIETY. [Continued from vol. xxxiv. p. 528.] June 20th, 1917.-Dr. Alfred Harker, F.R.S., President, in the Chair. HE following communications were read: THE 1. The Pre-Cambrian and Associated Rocks of the District of Mozambique.' By Arthur Holmes, A.R.C.S., D.I.C., B.Sc., F.G.S. Beyond the coastal and volcanic beds of Mozambique (described in a previous contribution-Abs. Proc. Geol. Soc. 1916, No. 994, p. 72) the country assumes the form of a gently undulating plateau, gradually rising towards the west and diversified by innumerable inselberg peaks and abruptly-rising clusters of hills. The dominant rock throughout is a grey biotite-gneiss. Interfoliated with this are occasional lenticular masses of hornblende-gneiss and amphibolite, and within these smaller bands of crystalline limestone are sometimes preserved. In many places the gneisses become garnetiferous, while eclogites and basic granulites also occur. Schists-referable to arenaceous sediments-are found only near the coast, where they are interbanded with gneisses; and, as the latter are mainly of igneous origin, they are thought to be intrusive into, and therefore younger than, the schists. As a general rule, the foliation and the banding of the gneisses are well defined in parallel uncontorted planes, the strike being commonly along, or somewhat north of, a north-east to south-west direction. In certain inselberg peaks, the strike sweeps round the contours, while the foliation-surfaces dip quaquaversally from the summits. Into the gneisses later granites, belonging to at least two different periods, have penetrated, riddling them with enormous numbers of small intrusions, lit-par-lit injections, tongues, and apophyses. Rocks of later age are rarely met with; but, in a few places, dykes of picrite and pyroxenite have been found cutting the youngest pegmatites. The succession of rocks in eight of the better-known districts is described, and the following general classification is based on the details thus provided: The above correlations of certain groups of rocks with the Lower and Middle Pre-Cambrian of other regions are based on the determination of lead-uranium ratios of zircons derived from the gneisses and granulitic granites respectively, the zircons having been obtained by crushing and panning the rocks in the field. The gneisses give a ratio of 0.21, comparable with a ratio of 0.24 obtained for Canadian zircons of Laurentian age. The granulitic granites give ratios of 0.14 to 0.17, comparable to those of radioactive minerals of late Archæan: that is, late Middle PreCambrian, age in Scandinavia (Moss 0·12 to 0·15, Arendal 0·16 to 0-18, and Ytterby 0.15 to 0.17), Canada (Villeneuve, Quebec, 0·17), and India (Singar 0.14). The rocks are described in detail, with tables giving the quantitative mineral composition and the specific gravities and radium contents. Numerous examples of contact-phenomena between crystalline limestones and various types of igneous rock are recorded: pyroxene, amphibole, sphene, and soda-lime felspar being the new minerals chiefly developed between granite and limestone, with garnet and scapolite also in special cases. With reference to the origin of the crystalline limestones and gneisses, the following conclusions are arrived at: : (a) The crystalline schists and limestones are interpreted as arenaceous and calcareous facies respectively of an ancient sedimentary series, their argillaceous complements being unrepresented unless they enter into the composition of the biotite-gneisses. (b) The limestones have controlled the formation of hornblende gneiss and amphibolite by their interaction with a granitic magma that elsewhere is represented by biotite-gneisses. The cores of the limestones have been enabled to resist further silicification by being thus enclosed within a blanket of rocks impoverished in silica. (c) If the ancient sedimentary series included argillaceous formations, it is thought probable that the gneisses are composite rocks produced by the concordant injection of granitic magma into such formations. This view, although not proved, is supported by mineralogical and radioactive evidence, and by the fact that in certain inselberg peaks the banding of the gneisses gradually dies away as the slopes are ascended, the rocks passing into granulitic granite nearly free from biotite and showing few traces of foliation. These peaks are interpreted as the irruptive foci of granulitic magmas which fed the lateral intrusions represented by the surrounding gneisses. It is shown that there are at least three types of inselberg peaks that owe their survival to peculiarities of structure and composition. The first type is that just mentioned, in which the foliation is less marked and the biotite-content appreciably lower than in the surrounding gneisses. In the second, the peaks are mainly composed of granulitic granite (again poor in biotite compared with the gneisses), and in the third type the peaks are riddled with tongues and apophyses of pegmatite Phil. Mag. S. 6. Vol. 35, No. 205. Jan. 1918. L and aplite. In each case the greater resistance offered to denudation is related to the presence of less foliated and more felsic rocks than are found in the adjacent plains. There remains a fourth type-perhaps the most abundant-in which no differences have been recognized. Many of these seem to be isolated relics of gneissic escarpments; and it is suggested that desert erosion, involving the attack of slopes at their base by arid weathering, and the removal of disintegrated material by wind, is the most favourable condition for the development and maintenance of an inselberg landscape. Existing conditions of denudation are considered to be unfavourable to inselberg survival; for the peaks appear to be worn down by the removal of superficial layers by exfoliation more rapidly than the surface of the plateau is lowered. 2. The Inferior Oolite and Contiguous Deposits of the Crewkerne District (Somerset).' By Linsdall Richardson, F.R.S.E., F.G.S. November 7th.-Dr. Alfred Harker, F.R.S., President, A Lecture on 'The Nimrud Crater in Turkish Armenia' was delivered by FELIX OSWALD, B.A., D.Sc., F.G.S. The Nimrud volcano, one of the largest volcanic craters in the world, is situated on the western shore of Lake Van, and was surveyed and investigated geologically for the first time by the speaker in 1898. The western half of the crater is occupied by a deep lake of fresh water, while the eastern half is composed of recent augite-rhyolites, partly cloaked in white volcanic ash. The crater-wall is highest on the north (9903 feet), rising in abrupt precipices over 2000 feet above the lake (7653 feet). The southern wall is also precipitous, but only reaches the height of 9434 feet (the south-eastern part). A large slice of the crater-wall has slipped down on the south-west, so as to form a narrow shelf, 800 feet above the lake. The crater is nearly circular, 8405 yards from west-south-west to east-north-east, while the transverse axis is 7905 yards. The lowest points lie on the long axis, reaching only 8139 feet on the western, and 8148 feet on the eastern rim. The crater-wall has an external slope of 33° on the south and east, where it consists exclusively of overlapping lenticular flows of augite-rhyolite and obsidian. On the south-west, west, north-west, and north these are capped by thin sheets of cindery basalt which must have possessed great fluidity, extending for many miles to form wide plains of gentle slope and great fertility down to Lake Van on the east and into the Plain of Mush on the west. These basalt-flows dammed up the north-east to south-west valley between |