Abstract
LONDON
Geological Society, May 24.—Prof. John Morris, Vice-President, in the chair. Messrs. Mosley, Colvin, Noble, F.R.A.S., and Davey, were elected Fellows of the Society. The following communications were read:-(1) “On the principal Features of the Stratigraphical Distribution of the British Fossil Lamellibranchiata.” By Mr. J. Logan Lobley, F.G.S. In this paper the author showed, by means of diagrammatic tables, what appears to be the present state of our knowledge of the general stratigraphical distribution of the fossil Lamellibranchiata in Britain. As a class, the Lamellibranchs are sparingly represented in the Lower, and more numerously in the Upper Silurian group, and fall off again in the Devonian; they greatly increase in number in the Carboniferous, become scanty in the Permian and Trias, and attain their maximum development in the Jurassic rocks. They are also largely represented in the Cretaceous and Tertiary series. The stratigraphical distribution of the two great subordinate groups, the Siphonida and the Asiphonida, corresponds generally with that of the class; the Siphonida predominate over the Asiphonida in Tertiary formations, whilst the reverse is the case from the Cretaceous series downwards. Nearly all the families of Lamellibranchs are represented in the Jurassic and Carboniferous rocks, and in the former very largely. The author remarked especially on the great development of the Aviculidæ in Carboniferous times. Mr. Etheridge, after noticing the importance of the paper, remarked that possibly the great difference observed in the proportions of Lamellibranchiata in different formations might to some extent be due to our want of knowledge. Of late years, in the Caradoc and Lower Silurian series, the number of species had been nearly doubled, principally through the persevering industry of one single observer, Lieut. Edgell. The same was to some extent the case in the Carboniferous rocks, owing to the collections of Mr. Carrington. Much was also being done for the Oolitic series, in connection with which the names of Mr. C. Moore, Mr. Sharp, and Dr. Bowerbank ought to be mentioned. Mr. Griffiths and the Rev. Mr. Wiltshire were doing the same work for the Gault. What the late Mr. S. P. Woodward had done as to the distribution of the different species of molluscs through time, Mr. Lobley was doing on a larger and more extended scale. Prof. Ramsay was glad to find that Mr. Lobley was, to some extent, doing the same for the Lamellibranchiata as Mr. Davidson had done for the Brachiopoda. He did not know how the case might be with the Silurian and Devonian formations, but in the Carboniferous strata the Lamellibranchiata were obtaining a preponderance over the Brachiopoda. He accounted for their comparative absence in formations of other ages, especially between the Upper Silurian and Rhætic beds, by the best known areas of those periods having been mainly continental, or containing principally freshwater or inland sea remains, so that the true marine fauna was absent. In Carboniferous times possibly the true relative proportions of the two forms had been preserved in the deposits. Mr. Judd was doubtful as to the safety of placing too great reliance upon figures. He questioned whether some of the conclusions as to the great increase of Lamellibranchiates between the Carboniferous and Jurassic periods could be substantiated. Much depended on the amount of the rocks present in different countries, and the study bestowed on each. The conditions also for the preservation of the fossils might be more favourable at one time than another. Mr. Carruthers considered the tables as of the greatest value, as indicating the present state of our knowledge. He called attention to the difference of conditions under which deposits had accumulated, which must have to some extent affected the proportion of Lamellibranchiates preserved in the different formations. Mr. Charlesworth remarked on the occurrence of Trigonia in the Australian seas, and on there being varieties of form among specimens of existing species so great that if they were found fossil they might be regarded as of several species. Mr. Hughes considered that the data were too incomplete to justify the generalisations of some of the previous speakers. It had been pointed out that whenever the tables showed a very large number of Lamellibranchs from any formation, that formation had been carefully worked out by local observers; and therefore he would like to know in each case the proportion the Lamellibranchiata bore to the total number of fossils found. It had been shown also that a larger proportion of Brachiopoda had been found in the older rocks, and of Lamellibranchiata in the newer. But in the older rocks whole genera of Lamellibranchs are confined to horizons and localities which are not cut off by stratigraphical breaks, such as would allow us to think it at all probable that they can be characterised by peculiar genera. He thought the scarceness and irregular occurrence of Lamellibranchs in the older rocks could be best explained on the supposition that those portions of the older deposits which were least favourable to Lamellibranchs happened to be those now chiefly exposed to our search, and that those few portions are only in part worked out. Mr. Jenkins observed that in thick deposits there was a far greater likelihood of numerous forms being present than in thin, for thickness meant time, and time meant variation. Prof. Morris dissented from this view, as in thin littoral deposits an enormous number of shells might be present, while in beds formed of deep sea they might be almost entirely absent—2. “Geological Observations on British Guiana,” by Mr. James G. Sawkins, F.G.S. In this paper the author gave a general account of his explorations of the Geology of British Guiana when engaged in making the geological survey of that colony. He described the rocks met with during excursions in the Pomeroon district, along the course of the Cuyuni and Mazuruni rivers, on the Demerara river, on the Essequibo and its tributaries, on the Rupununi river, and among the southern mountains. The rocks exposed consist of granites and metamorphic rocks, overlain by a sandstone, which forms high mountains in the middle part of the colony, and is regarded by the author as probably identical, or nearly identical, with the sandstone stretching through Venezuela and Brazil, and observed by Mr. Darwin in Patagonia. Prof. Ramsay remarked upon the barrenness, from a geological point of view, of the district investigated by Mr. Sawkins, and especially called attention to the absence of fossils in the stratified rocks. He referred briefly to Mr. Sawkins's labours in Trinidad and Jamaica, and to his discovery of metamorphosed Miocene rocks in the latter colony exactly analogous to the metamorphic Eocene rocks of the Alps. He was glad to see that the author had brought forward examples of cross-bedding in metamorphic rocks, and considered that the results adduced were favourable to those views of the metamorphic origin of granite which he had himself so long upheld. Mr. D. Forbes, on the contrary, considered that the facts brought forward by Mr. Sawkins were confirmatory of the eruptive nature of the granites observed. He added that cross-bedding was common in igneous rocks and even in lavas. Mr. Tate remarked that in the country to the north of the district described in the paper metamorphic rocks abound. He considered that the series of metamorphosed Jurassic rocks extends across the whole north of South America, and perhaps into California. Similar sandstones to those described occur in the basin of the Orinoco, and contain fossils which show them to be of Miocene age. Mr. Tate did not consider these sandstones as the equivalent of the Patagonian sandstones, as from the shells contained in the latter they would appear to be Pliocene or Pleistocene. Mr. Sawkins, in reply to a question from Mr. Tate, stated that the only gold found in the country had probably been carried down from the well-known gold district of Upata. He also entered into a few additional details connected with the chief points in his paper, dwelling especially upon the physical features of the country, in illustration of which several landscape drawings were exhibited.
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Societies and Academies . Nature 4, 115–116 (1871). https://doi.org/10.1038/004115a0
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DOI: https://doi.org/10.1038/004115a0