GEOLOGY — WILLIS. 209 



of square miles of land area. Whatever mountains had previously 

 existed had been eroded and continents, worn down to plains, sank 

 and were submerged. Strata deposited in those wide seas in Amer- 

 ica and Eurasia were to a great extent limestone ; terrigeneous sedi- 

 ment was not delivered in any quantity during a very long period ; 

 the physiographic aspect of surviving lands was that of the Siberian 

 plains, the lowest, the most monotonous ; there were scarcely any 

 mountains, so far as we know the continents. 



In reference to climatic conditions we may interpret the chemical 

 and biological characters of strata in terms of aridity and cold or of 

 humidity and warmth. Chamberlin has pointed out the way in his 

 papers on the grouping of red beds, saline deposits, and desert con- 

 glomerates in distinction to those sediments in which the oxides 

 were reduced in consequence of the presence of vegetation, or in 

 contrast to great limestone deposits from warm seas teeming with 

 life. That such grouping occurs in repeated sequences from pre- 

 Cambrian to the present is established ; the criteria still demand 

 much study, and there are problems, such as the surprising deposits 

 of gypsum in Oklahoma, for instance, for which, as yet, no adequate 

 explanation is advanced. The last word has by no means been said 

 in simple stratigraphy, and in this comparative study it is part of the 

 task to develop the residual questions. 



Thus far in discussing interpretation we have suggested only indi- 

 vidual formations, which are taken as illustrations of fixed condi- 

 tions ; but fixity is temporary in paleogeography, constant change 

 is the law for longer time divisions, and successive conditions are 

 recorded in sequences of strata. Where an individual fact or group 

 of facts is unintelligible when isolated, it may be understood readily 

 when placed in relation to those which went before. For instance, 

 the volume of vein quartz represented by the sand grains and pebbles 

 of the Carboniferous sandstones, either in Europe or the United 

 States, could not have been derived in any one epoch of erosion from 

 the gneisses which were its source. But this concentration of quartz 

 follows upon strata which are littoral or off-shore deposits, yet are 

 without those coarser beach sands that must have been formed near 

 shore contemporaneously with them. While these earlier formations 

 were accumulating, a coastal plain was the locus of concentration of 

 the relatively indestructible quartz, and this concentrated material 

 ultimately went to form the great mass of sandstones, which accu- 

 mulated in the Carboniferous basins. Thus the coastal-plain deposits 

 of Silurian and Devonian continents are in part found resting upon 

 strata of those ages in that conformable sequence in which they were 

 redeposited in consequence of geographic changes and erosion. 

 14 



