Rev. J. F. Blake — Form of Sedimentary Deposits. 73 



make up the bulk of the formation. We may quote the Ordovician, 

 Silurian, and Devonian slates, the Keuper, Lias, Oxford and 

 Kimmeridge Clays, the Gault and London Clay ; but we can give 

 ■no such list of thick masses of marine sandstone. Fine sediment, 

 therefore, has, as a matter of fact, made thicker masses of rack than 

 coarse sediment ; but this could not be the case if deposits thinned 

 out seawards, where the fine sediment is carried. Again, it is 

 impossible to imagine a thickness of a thousand feet and more 

 constantly occupying a position near the shore ; there is no room for 

 it. If you depress the land, you remove the shore. To draw 

 a diagram of one such deposit, an unnaturally scooped-out shore 

 has to be assumed, and it is impossible to repeat the process. 

 Indeed, a shape such as Fig. 2 is the only one possible for an 

 indefinite number of deposits on a sloping sea-bottom. There is 

 nothing, of course, to prevent a thinning out beyond the maximum, 

 and a film from most terrigenous deposits does, no doubt, spread 

 in that direction. 



And if we go into details we find that where a thick deposit 

 of fine sediment thins, we get independent evidence of an approach 

 to a shore, but none where it remains thick. 



Take the Cambrian and Ordovician strata as a whole. It has 

 been found necessary by those who assume that a thickening of 

 strata indicates approach to a source of supply to postulate a large 

 continent occupying the position of the north part of the present 

 Atlantic Ocean during those early times. But Mr. C. D. Walcott, 

 the Director of the U.S. Geological Survey, writing on "The North 

 American Fauna during Cambrian Time," ^ states that " There is 

 certainly no evidence to show that since the beginning of Palj^ozoic 

 time the beds of the deeper seas have been elevated above the 

 surface of the waters"; also that "The interior continental area 

 [of North America] was outlined then ["at the beginning of 

 Cambrian time"], and it has not changed materially since," and 

 he draws a diagram accordingly, showing the Cambrian strata 

 thickening as they leave the shores of this Pre-Cambrian continent. 



On this side of the Atlantic we learn from C. Schmidt ^ that 

 in the Baltic provinces the whole of the Ordovician strata are 

 represented by a thickness of 420 feet, whereas the same strata in 

 this country are said to exceed 12,000 feet. Much of the latter may 

 be of volcanic origin, but half the thickness would serve the present 

 purpose. Now, speaking of the Scandinavian Ordovicians on the 

 opposite side only of the Baltic, J. E. Marr ^ says : " Many of them 

 [the fossils] are found in shallow water beds, deposited at periods 

 when the greater part of the North-Western European area was 

 covered with deep water " ; and the fossils are certainly more 

 abundant than in this country. 



But these comparisons are on too large a scale. Let us take the 

 case of a deposit laid down in a single basin and during a more 



1 U.S. Geol. Survey Ann. Rep., vol. xii, p. 563. 

 - Q.J.G.S., vol. xxxviii, p. 513. 

 3 Q.J.G.S., vol. xxxviii, p. 513. 



