SEDIMENTATION, ENVIRONMENT, AND EVOLUTION. 19 



living marine mollusca. In the Carboniferous Period which 

 followed, deposits have been styled fresh- water or marine 

 according to the characters of their mollusca, and it is evident 

 that at least in these periods, if not earlier, the mollusca had 

 adapted themselves to a fresh-water or a marine environment. 



(5) PalcBogeographical conditions. Much light may be 

 thrown upon the ancient geography of the period by a study 

 of the derived fragments and minerals in rocks, and by the 

 variation in size and proportion of the constituents. The 

 penological character of the adjoining land- area, its relative 

 altitude, and the presence or absence of large trunk rivers, may 

 often be adduced from the character of the sediments. Ancient 

 deltas have thus been traced. Judging from the evidence 

 obtained by the " Challenger " expedition on the formation 

 of glauconite, the presence of that mineral in rocks is pre- 

 sumptive evidence of oscillatory marine conditions, on 

 the continental edge of great ocean basins where currents of 

 different temperature or salinity mingle, and possibly near the 

 mouths of rivers bringing little suspended matter into the 

 ocean, but a considerable quantity of dissolved salts. The 

 existence and direction of beneficial food-bearing or inimical 

 mud-bearing currents may also be indicated. 



(6) Diastrofkism. The evidence yielded by sediments of 

 secular movements of the earth's crust, slow or fast, was 

 considered in some of its aspects in the earlier part of this 

 address. The enormously-increased denudation consequent 

 upon considerable elevation is reflected in the vast accumula- 

 tions of the Flysch and Mollasse which fringe the Alps and 

 represent the wreckage of a great thickness of that mighty 

 mountain mass after the earlier stages of its uplift. On the 

 other hand, the depression of the British area which gave rise 

 to the great Cenomanian transgression was followed by the 

 formation of the Chalk in a basin which subsided at least 

 1,000 feet during its deposition. The remarkably purity of 



