difference of about 2 inches of rain may be expected betwixt years of greatest and least spot-area. This result is derived from observations at ten stations, distributed over a very small patch of the earth's surface; and it is evident that for any serious investigation a much larger series of observations representing the rainfall over a great extent of country would be essential. I now examined observations made at different stations in India; but this examination showed the extreme difficulty of obtaining a satisfactory result, either way, from a few stations in that country, when, in certain years, the accidental excess of rainfall at some of the stations may be 40 inches, even though deficiencies at some stations may diminish the amount of the error. From my own experience of rainfall on the Indian ghats, I should doubt that a mountain-station, such as Mussoorie, is well fitted to be employed in this discussion. If a single station could be taken to represent any tract of country, it ought to be one least liable to local causes of variation. Among the mountains a slight change in the average direction of the wind will cause great differences in the rainfall at stations but little distant from each other, and to eliminate accidental variations of 40 or 80 inches would require observations during a very long series of years. The following Table will, however, show the quantities which may have to be dealt with at an Indian hill-station † :— Values of AR for Mahabuleshwar, 4500 feet above the sea, or that 26 inches more rain falls for the year of least than for For the rainfall at Mahabuleshwar, see Colonel Sykes's paper on Indian observations, Phil. Trans. 1850, p. 367. The mean fall is 2530 inches. * Indicates opposite signs of AA and AR. that of greatest spot-area. The examination of many series of observations has shown how difficult it will be to arrive at a conclusion for a quantity so small as 2 inches of rain. It is evident that a larger tract of country than Great Britain should be chosen, and the approximate rainfall be deduced from the greatest possible number of stations. Germany and France may give sufficient data for such a trial. Were the result well. marked, there would be reason to seek for its confirmation in other countries; but to undertake this labour, better grounds, I think, must be found than I have hitherto been able to obtain. The admirable series of observations which Mr. Symons is obtaining will suffice for the future, as for the past, ten years to give a very near approximation to the excess or deficiency of rainfall in Great Britain. GEOLOGICAL SOCIETY. [Continued from p. 410.] June 24, 1874.-John Evans, Esq., F.R.S., President, In the Chair. The following communications were read: 1. "New Carboniferous Polyzoa." By Prof. John Young, M.D., F.G.S., and John Young, Esq., F.G.S., Hunterian Museum, Glasgow University. The authors described two new forms of Polyzoa from the Carboniferous Limestone shales near Glasgow. For one of these they proposed to form a new genus, Actinostoma, allied to Fenestella, with the frond erect, and poriferous on one face only, and with the fenestrules oblong. The species they named A. fenestratum. The second species was described under the name of Glauconome stellipora. 2. "On Palæocoryne and other Polyzoal Appendages." By Prof. John Young, M.D., F.G.S., and John Young, Esq., F.G.S., Hunterian Museum, Glasgow University. The authors maintained that the structures described by MM. Duncan and Jenkins under the name of Palæocoryne, are not independent organisms, but mere processes of the Polyzoa on which they occur, the cells at the base being only the cells of the Polyzoa. They stated that the tissues of the two structures are perfectly continuous, that the "dactylose basal lobes" are the branches of the Polyzoon of which the processes are continuations, that the stem, capitulum, and branches of the processes are solid, that there is no normal aperture in the capitulum, that the stems do not always bear capitula, even when they give off branches, and that in many cases they are mere processes without either capitulum or branches. According to their observations, the stellate processes called Palaocoryne are given off chiefly from the poriferous face, and the other processes from the poriferous and non-poriferous faces, and from the edge and base of the frond. The authors did not express any positive opinion as to the function of these processes; but they suggested that those given off from the non-poriferous face and from the base of calycine fronds, may serve as adventitious roots, and those given off from the margins and from the poriferous face near the margins may be irregularities of growth, unless they are the commencement of new fronds. 3. "The Steppes of Siberia." By Thomas Belt, Esq., F.G.S. The author described the portion of the Siberian steppes traversed by him as consisting of sand and loam. The best section seen by him was at Pavlodar, where he found 1 foot of surface soil, 20 feet of stratified reddish-brown sand, with lines of small gravel, 8 feet of light-coloured sandy silt, 15 feet of coarse sand, with lines of small pebbles and 1 line of large ones, and 6 feet of clayey unlaminated silt, with fragments of the bed rock in its lower half, the bed rock being Magnesian Limestone much crushed at the top. South of Pavlodar the surface was covered with pebbles, which became larger in advancing southward, until the soil was full of large angular quartz boulders. Further south the bed-rock comes to the surface in ridges and low hills, increasing in height until some of them attain 2000 feet. All the rock-surfaces were much shattered, as if by the action of frost, but they showed no signs of glacier-action. The ridges and hills were separated by plains composed of sandy clay, with numerous angular fragments derived from the rocks in the immediate neighbourhood. This is accounted for by the author on the supposition that they formed a series of shallow lakes, frozen over in winter, and that the ice on breaking up carried away fragments of the rocks. The distribution of the boulders on the plain north of the ridges was also attributed to floating ice. The generally accepted marine origin of the great plain was said to be negatived by the absence of sea-shells in its deposits, whilst Cyrena fluminalis occurs in them. The author regards them as deposits from a great expanse of fresh water kept back by a barrier of polar ice descending far towards the south. In its greatest extension this ice-barrier would produce the crushing of the bed-rock; and as it retreated, the water coming down from the higher ground in the south would cover a continually increasing surface. 4. "On the Microscopic Structure and Composition of British Carboniferous Dolerites." By S. Allport, Esq., F.G.S. The object of this paper is to supply further and conclusive evidence to show that there are dolerites and basalts of Carboniferous age whose original mineral constitution is precisely the same as those of the later Tertiary periods, those of both ages presenting the same varieties of structure, and that the great alterations which most of the older rocks have undergone constitute the only difference between the two groups. The author describes at some length the various constituents under the following heads, viz. felspar, augite, olivine, magnetite, mica, apatite, glassy matrix, &c. He next describes the occurrence of dolerites in the Midland Coalfields, Ireland, Edinburgh, Arran, &c. In conclusion he draws attention to the many variations in composition and texture in the same rock-mass, and accounts for them thus: If the lava were simply in a viscid state, with the ingredients imperfectly mixed, portions of it must, on consolidation, contain them in various proportions, just as is known to be the case in imperfectly fused slags. He maintains that there is an absolute identity of composition, structure, and mode of occurrence in these eruptive rocks of very widely separated geological periods, and that therefore they should be placed in one group. 5. "Additional Remarks on Boulders, with a particular reference to a group of very large and far-travelled erratics in Llanarmon parish, Denbighshire." By D. Mackintosh, Esq., F.G.S. The author, after referring to a number of northern-drift boulders in addition to those he had noticed in a former paper, describes several large felspathic boulders found up to a height of about 1750 feet above the sea, on Cefn-y-fedw, N. of Llangollen. He then gives a somewhat detailed account of the drifts in the neighbourhood of Corwen, and of some large felspathic boulders, probably from the Arenig mountains, which are generally found on the surface, or interposed between the lower boulder-clay and an upland extension of the middle sand of the plains. The main part of the paper is devoted to an account of the discovery of a numerous group of very large and far-travelled felspathic boulders in the parish of Llanarmon, Denbighshire. The author refers particularly to a remarkable slickensided boulder, and to the great "Immovable Stone" at Maendigychwyn (now called Eryrys), about 1150 feet above the sea, which is the largest far-transported boulder he has heard of in the British Isles. He stated a number of facts and considerations which led him to believe that the Llanarmon boulders, along with those further N. and W., must have come all the way from Snowdon, and that they were floated over passes or cols in the intervening ranges of hills by icebergs or coast-ice about the close of the Lower Boulder-clay period. He concluded by noticing the necessity for a personal examination of boulders instead of relying on answers to queries, and stated that about Llanarmon the felspathic boulders are called "granite tumblers," while in Cheshire all kinds of boulders are called " marble stones." 6. "Note on the Bingera Diamond-fields." By Archibald Liversidge, Esq., F.G.S. The author commenced by describing the general characters of the older Australian Diamond-field of the Mudgee or Cudgegong District. The Bingera Diamond-field is situated in a basin among the mountains of the Drummond range, the encircling hills being of Carboniferous or Devonian age. The diamantiferous drift occurs in patches in the basin, which is invaded by spurs of basalt. The |
