UNCONFORMITY INFERRED FROM FOSSILS. 341 



and the Longmynd, on contorted and denuded Llandeilo and Cambrian 

 rocks. The Upper Llandovery contains 91 genera and 261 species, of 

 which 59 genera and 126 species pass up to the Wenlock. There is 

 no clear and satisfactory unconformity of a visible kind here, though 

 the Upper beds of the Llandovery series are sandstones and conglome- 

 rates indicative of shallower water conditions than those of the typical 

 succeeding Wenlock beds. The Wenlock contains 536 species of 

 fossils, of which 126 pass into the Lower Ludlow. The Ludlow 

 fauna includes 137 genera and 392 species, and the beds have 129 

 species in common. 



There is a complete break in life between the Silurian and over- 

 lying Devonian series, but there is no sign of unconformity or a break 

 of any kind between the Ludlow rocks and the Old Red Sandstone. 

 Yet only 20 species pass up into the Devonian, which is about one- 

 thirteenth of the species in the Upper Ludlow rocks, and less than 

 one-third of the fauna of the passage-beds. The Devonian rocks con- 

 tain 195 genera and 544 species, of which 32 genera and 51 species 

 pass up to the Carboniferous ; but Sir Andrew Ramsay observes that 

 round the Forest of Dean and the South Wales coal-field there is no 

 sign of unconformity between the Old Red Sandstone and the Car- 

 boniferous series. Yet, although conformable to the strata above and 

 below, the Old Red Sandstone includes distinct unconformities in 

 Scotland. 



The Carboniferous series is generally conformable from top to 

 bottom, but the beds exhibit many oscillations of level. In the 

 Forest of Dean, the Millstone Grit rests unconformably on the moun- 

 tain limestone, but there is nowhere a gap that would correspond to 

 the change in life. The fauna comprises no less than 515 genera and 

 2409 species, of which only 51 species are derived from the Devonian, 

 and only 8 species pass up into the Permian. 



The Permian rocks show a marked unconformity resting on all 

 the Primary strata. ,The fauna is poor, and, so far as Brachiopoda are 

 concerned, is largely composed of species which live on from the 

 Carboniferous. Thus there appears to be a more marked physical 

 distinction between these strata than suggests itself on an actual 

 comparison of Permian with Carboniferous fossils ; and we find that 

 the physical break is not coupled with a pala3ontological break. 



Breaks in the Secondary Strata. We now come to the great 

 stratigraphical break which marks the commencement of the New 

 Red Sandstone ; and nowhere is there an actual passage downwards 

 into the Permian. The Triassic rocks are very imperfectly represented 

 in this country, and the series includes some unconformities, since 

 near Ormskirk the New Red Marl lies unconformably on the New 

 Red Sandstone. Only one Trias species appears to range into the 

 Lias. There is no appearance of unconformity between the New Red 

 Marl and the Rhsetic beds, although the latter are a marine series. 



The Lias is so closely connected with the Rhsetic beds, that the 

 separation between them has only been made during the last twenty 

 years, and there is no visible unconformity in the Lias ; but the per- 



