472 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



British Isles are found areas of "Old Eed sandstone" — in part at least 

 land and lake deposits — of great thickness, that are commonly believed 

 to represent several distinct basins. Erosional unconformities occur 

 within these deposits, showing that considerable, though perhaps local, 

 changes of level were taking place. 



While warping and local emergences occurred rather frequently during 

 the Devonian in America as in Europe, it is yet a fact that, on the 

 whole, this was a period of progressive overlap. The Helderbergian or 

 lower Devonian deposits everywhere are overlapped by the middle 

 Devonian and this, in many places at least, by the late Devonian. In 

 eastern America the lower Devonian contains much less clastic matter 

 than in west central Europe, but that would seem to be due to the fact 

 that whereas in America the Helderbergian invaded late Silurian lime- 

 stone areas, from which but little clastic sediment could be derived, in 

 Europe the early Devonian sea advanced over a much older and probably 

 soil-laden land. The middle and upper Devonian deposits are essentially 

 similar in the two continents, the former consisting chiefly of limestone, 

 the latter of sandstone and shale in western Europe and eastern America 

 and mostly of limestone in the more inland areas. In both America and 

 Europe the lower Devonian was deposited in old downwarps which from 

 time to time in the preceding Paleozoic ages had been accentuated. 

 These downwarps suffered little erosion, the contact between the base 

 of the lower Devonian and the underlying rocks being, therefore, as a 

 rule not conspicuously unconformable. But the upwarped boundaries of 

 the old troughs or basins: these were eroded; hence where the middle 

 Devonian overlaps on them marked unconformities prevail. 



It is fairly well established that no very important deformation 

 occurred at the close of the Silurian and the beginning of the Helder- 

 bergian Devonian. The character and distribution of the sediments of 

 the latter indicate oscillating though on the whole gradual subsidence of 

 intramarginal troughs, like the Appalachian Valley, and corresponding 

 slowly advancing submergence of such troughs. Prior to this time, 

 however, occurred the important late Ordovician-Silurian emergence, 

 which, strange to say, was revived and attained considerable dimensions 

 in the Appalachian Valley during the middle Silurian (late Niagaran). 

 This statement is based on the fact that, although the Clinton (including 

 the Eochester shale) is well represented in this valley, no trace of later 

 Niagaran deposits has been found there. It is to be noticed, however, 

 that the middle Silurian emergence of the Appalachian Valley referred 

 to probably was not accompanied by disproportionate elevation of the ad- 

 jacent areas which had contributed the clastic material of the preceding 



