216 JELenry Hicks — On the Northern Palceozoic Rochs. 



of loose material was ready at hand, it is eyident that the change^ 

 must have been on the whole rapid, and that a depression of the 

 crust even in those parts again soon followed that of elevation. The 

 elevation, as already stated, occurred onlj'- over a very small propor- 

 tion of the Northern Hemisphere, compared with that in which 

 depression was going on. The subsidence also in the larger areas 

 continued to go on uninterruptedly at this point, and we find 

 therefore that the beds which can be considered in any way the 

 equivalents of the Lower Llandovery rocks of Wales in these un- 

 disturbed areas are very insignificant, and usually have no well- 

 defined boundary.^ I know that the generally received opinion is 

 that these changes took place very gradually ; but I think the 

 above evidence tends strongly to show that elevation as a rule was 

 rapid, compared with that of depression; and I further believe 

 that it was so in all the changes which took place up to the close 

 of the Palaeozoic. The sediments which make up the earlier Upper 

 Silurian rocks in the neighbourhood of the uplifted parts are such 

 as would result from the denudation of these, and hence all con- 

 ditions may be seen from those of beach pebbles to the very fine 

 muddy sediment deposited at a distance from land. The Upper and 

 Lower Llandovery rocks (including the May Hill Sandstones and 

 Denbighshire Grits) show this clearly, and any variation in thick- 

 ness or in composition is due to this cause. In the parts which 

 remained submerged, as Pembrokeshire, Carmarthenshire, and prob- 

 ably Cardiganshire in South Wales, the sediments form a con- 

 formable series from the Lower to the Upper Silurian, whilst in 

 the immediately disturbed areas an unconformity might be seen at 

 any point up to the close of the Upper Silurian. 



The higher groups in the Upper Silurian are made up of sedi- 

 ments, formed during periods in which a gradual depression of the 

 sea-bottom was generally taking place. In Britain they are com- 

 posed of an admixture of argillaceous material and of limestone. In 

 the immediate regions of the parts at this time above water, but little 

 limestone occurs, the rocks being made up almost entirely from the 

 denudation of these islands. In France and Spain these groups are 

 scarcely represented, but in Bohemia, Norway, Sweden, Eussia, and 

 in parts of Asia, they are represented chiefly by limestones. In 

 North America by limestones, in the western and undisturbed 

 regions, and partly by schists and sandstones along the eastern areas, 

 or in the parts which had been disturbed at the close of the Lower 

 Silurian (see Table, p. 156). The Upper Silurian period, therefore, 

 was one of continued and gradual subsidence in apj^arently most 

 of the European, and of the North American, and probably also of 

 the Asiatic areas. The thickness of the series varied of course greatly 

 in different regions, and the composition also, as we have shown, in 

 proportion to the local changes which took place. The lines of 



^ The following facts concerning the fossils, kindly communicated to me by Mr. 

 Etheridge, F.R.S., strongly support this view of rapid deposition for the Llandovery 

 rocks. Of tlie 143 species wliich occur in the Lower Llandovery in Britain, 17 only 

 can be said to be restricted to that formation ; and of the 236 species wliich have 

 been found in the Fpper Llandovery, 47 only are peculiar to that series. 



