94 Geologists' Association. 



Prof. Williamson commented on the difficulties of the case, and said he believed 

 that in most fossils more than one process had conduced to the result. He agreed with 

 Mr. Charlesworth as to the character of the Yorkshire fossil sponges, in which the silica 

 had most completely replaced the keratose fibres. By the action of acid it was 

 possible to obtain their skeletons as perfect as those of living species. There had 

 been no mere infiltration in the case, but a real chemical union between the silica 

 and the keratose fibres. At Flamborough Head, close by silicified sponges, he had 

 found others in which the organic matter had been entirely replaced by oxide of 

 iron instead of silica; and these, on being treated with acid, were dissolved. In 

 the case of the foraminifera, three distinct agencies had taken place, — one an 

 infiltration, another the conversion of animal matter into flint, and the third that of 

 calcareous matter into flint also. 



Mr. Sollas, in reply, pointed out that he had not stated that flint was formed 

 by the silicification of sponge-tissue, but by] that of animal matter. He did not 

 think that any mere mineral change could account for the forms of flints. He had 

 only dealt with a limited portion of the question of the origin of flint and coprolites. 

 Of the organic origin of the latter there could be no doubt. He thought that the 

 non-silicified condition of recent dead sponges might be due to a defective supply of 

 silica in the sea-water. He had not put so much stress on dialy&is as had been 

 supposed, but relied mainly on the deposition of flint by means of organic matter. 

 From the presence of siliceous spicules of peculiar forms in the coprolites, he could 

 not accept them as of Alcyonarian origin. 



Geologists' Association — January 3, 1873. — The Eev. T. "Wiltshire, M.A., 

 F.G.S., President, in the Chair. — " On the Cambrian and Silurian Rocks of 

 Eamsay Island, St. David's," by Henry Hicks, Esq., F.G.S. 



In an exposed coast section which occurs at the north end of Eamsay Island, 

 the three important groups of strata known under the names Lingula Flags, 

 Tremadoc, and Arenig Groups, are seen resting on one another in the order of 

 their succession, and probably are better exhibited than at any other place in 

 "Wales. The first two groups are those now usually recognized as forming the 

 upper part of the Cambrian; and the latter as the lowest group of the Silurian 

 system. This section is therefore of considerable importance, in its bearing on 

 classification, as it shows clearly the relation of the groups to each other. The 

 Lingula Flags occur as hard siliceous sandstones with grey flaky slates, and dip 

 under the others at an angle of about 60 degrees. They contain the usual shell, 

 Lingulella Bavisii, in great abundance ; also a Trilobite of the genus Neseuretus, 

 Eophyton, a supposed land-plant, and numerous worm tracks. The beds are frequently 

 ripple-marked, and give indications of having been shore or shallow water accu- 

 mulations. The Tremadoc group rests quite conformably on the Lingula Flags, 

 and at first the beds are much like those of the latter in their lithological characters, 

 but afterwards they gradually assume a darker and more flaggy appearance. Fossils 

 are very plentiful in these beds, and numerous new forms come in. Amongst these 

 may be mentioned the Lamellibranchs, Star-fishes, and Encrinites. The Trilobites 

 belong to the genera Niobe and Neseuretus. A gentle and gradual depression of the 

 sea-bottom was evidently taking place during the deposition of this group. Eesting 

 upon the last mentioned is the Arenig group, a series of black iron-stained slates 

 and flags, and with a fauna wholly distinct from that of the Tremadoc group. The 

 Graptolites come in here for the first time, as well as the genera JEglina, Trmucleus, 

 and Ogygia. In many respects the fauna resembles that of the Quebec group of 

 Canada. For the deposition of these beds a deep and decided depression of the sea- 

 bottom must have taken place, and if the succession here is unbroken this must have 

 been sudden. It is probable, however, that a fault has passed along the strike of 

 the beds, and that this has removed the series which should have intervened to 

 connect them more closely, lithologically, and palaeontologically. As far as can be 

 made out by this section, tbe boundary line between Cambrian and Silurian should 

 be certainly placed above the Tremadoc group as exhibited at St. David's (the 

 upper part of the Tremadoc group of North Wales will doubtless have to be 

 included in the Arenig group), and below the Arenig group. 



