284 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EAETH. 



Let US look at the analyses of the '' roofing slates." In these a cer- 

 tain fineness of grain and attendant similarity of history are probal)ly 

 in most cases involved. These are types mainly of the finest sediments. 

 It does not appear that we have any reason to suppose that their depo- 

 sition, consolidation, and prolonged existence in the rocks added to or 

 subtracted from their original chemical constituents. With these we 

 may probably compare- clay slates of more modern periods and the 

 finest nmds now being laid down in estuaries and lakes. 



Referring to Clarke and Hillebrand's collection of rock anah'ses/ we 

 find sixteen analyses of roofing slates of Cambrian age from Vermont 

 and New York. The mean percentage of added potash and soda alka- 

 lies is 5.05. In Rosenbusch (loc. cit., p. 425) the alkalies in a Welsh 

 roofing slate are recorded as 5.38 per cent; a Cambrian clay slate of 

 the Fichtelgebirge, 5.53 per cent; a Lower Silurian clay slate of the 

 same region, 4.10 per cent, and a Silurian clay slate from Christiania, 

 5.60 per cent. These are otherwise nuitually fairly concordant in 

 chemical composition, and also concordant with those from the United 

 States. The mean of all these affords 5.08 per cent of alkalies, the 

 potash in each case exceeding the soda. 



In this same table of Rosenl)usch's we find a Devonian roofing slate, 

 Erbstollen, with 3.04 per cent of alkalies. Three other Devonian 

 slates, not named as roofing slates (Nos. 2, 3, and 4), show a mean of 

 3.54 per cent. 



In the culm we find a roofing slate having 3.22 per cent; another 

 culm roofing slate, 5 per cent, and an upper culm gr&y clay slate of 

 the Fichtelgebirge, 2.99 per cent. 



If we compare with these ancient sediments those now being 

 deposited, we obtain the following figures: Bischof records 1.47 per 

 cent of alkalies in the suspended matter carried down by the Rhine 

 near Bonn.*^ Although this is fast-moving water, the general analyses 

 otherwise closely resemble roofing slates and clay slates, as Bischof 

 points out. The mud of the Nile near Cairo afiords 1.96 per cent.^ 

 Merrill gives two analyses of fine muds washed by the sea into harbors 

 and bays on the coast of North Carolina. They are fine, dark-colored 

 muds brought down by the rivers and mixed with some deca3ang ani- 

 mal and vegetable matter. These contained 1.97 per cent and 2.17 per 

 cent of alkalies; or, deducting all organic matter and water (which are 

 temporary constituents), these numbers rise to 2.37 and 2.39 per cent. 

 The mean given by these four modern silts and muds is 2.05 per cent 

 of alkalies. 



Without further investigation the facts recorded can only be advanced 

 as suggestive. 



1 Bulletin TJ. S. Geological Survey, No. 148, 1897. 

 ■'Chemical and Physical Geology, p. 123. 

 ^Loc. cit.,p. 133. 



