574 THE POPULAR SCIENCE MONTHLY. 



Sandstone, as distinct from the marine Devonian rocks, is only occa- 

 sionally and hesitatingly allowed to have a fresh-water origin, in spite 

 of the statement made by Mr. Godwin-Austen long ago, that it was 

 deposited in lakes. 



My present object is to prove that, in the British Islands, all the 

 great formations of a red color, and which are partly of Palceozoic, 

 and partly of Mesozoic or Secondary age, were deposited in large in- 

 land lakes, fresh or salt ; and if this can be established, then there 

 was a long continental epoch in this part of the world comparable to, 

 and as important in a physical point of view as any of, the great con- 

 tinents of the present day. 



The Upper Silurian rocks of Shropshire, Herefordshire, Monmouth- 

 shire, and South Wales, are succeeded immediately by the Old Red 

 Sandstone series, and there is no unconformity between them. 



The teeming life of the Upper Silurian seas, in what is now "Wales 

 and the adjoining counties, continued in full force right up to the nar- 

 row belt of passage-beds which marks the change from Silurian brown 

 muddy sands into lower Old Red Sandstone. In these transition beds 

 on the contrary, genera, species, and often individuals, are few in num- 

 ber and sometimes dwarfed in size, the marine life rapidly dwindles 

 away, and in the very uppermost Silurian beds land-plants appear, 

 consisting of small pieces of undetermined twigs and the spore-cases 

 of Lycopodiacese {Pachytheca spherica). Above this horizon the 

 strata become red. 



The poverty in number and the frequent small size of the shells in 

 the passage-beds indicate a change of conditions in the nature of the 

 waters in which they lived ; and the plants alluded to clearly point to 

 the close neighborhood of a land, of which we have no direct signs, in 

 the vast development of a purely marine fauna in lower portions of 

 the Ludlow strata. In the Ludlow bone-beds the fish-remains, Onchus 

 and Sphagodus, and the large numbers of marine Crustacea, almost 

 entirely trilobitic in the Ludlow rocks, indicate a set of conditions 

 very unlike those that prevailed when the passage-beds and the lower 

 strata of the true Old Red Sandstone were deposited, in both of which 

 new fish appear, trilobites are altogether absent, and are more or less 

 replaced by Crustacea of the genera Pterygotus and Eurypterus, one 

 of which, Eurypterus Symondsii, has only been found in the lower 

 Old Red Sandstone. Neither are there any mollusca in the Old Red 

 Sandstone, excepting where that formation passes at the top into the 

 Carboniferous rocks. All these circumstances indicate changes of con- 

 ditions which were, I believe, of a geographical kind, and connected 

 with the appearance in the area of fresh water. 



The circumstances which marked the passage of the uppermost 

 Silurian rocks into Old Red Sandstone seem to me to have been the 

 following : First, a shallowing of the sea, followed by a gradual alter- 

 ation in the physical geography of the district, so that the area became 



