GEOI.OGY — CHAMBERLIN, l8l 



waters, this is not well supported by physiographic and fauna! evi- 

 dences. The lateral shear of a relatively thin shell, whose thickness 

 is a function of rock pressure and rigidity and which hence has an 

 undulatory base rudely concentric with the surface of the lithosphere, 

 provides a mode of emergence and submergence by oblique movement 

 of the shell, without involving much massive movement of the conti- 

 nents beyond the shrinkage concerned in general deformation. This 

 explanation compasses at once the rejuvenation of the streams back 

 from the coasts, so markedly expressed in falls and rapids and the sub- 

 merged valleys off the coast, which seem to be coordinate phenomena 

 and common to all coast tracts. 



II. Ci.iM\Tic Oscillations. 



Another chief line of study lay in the endeavor to work out, as a 

 sequence of a planetesimal origin of the earth, a working system of 

 hypotheses relative to atmospheric states and their expression in the 

 climates of geological history. In my previous report the origin and 

 general history of the atmosphere, involving a postulated system of 

 secular supply and depletion, were outlined (Year Book No. 3, pp. 

 233-238). The supreme test of all atmospheric and climatic doc- 

 trines seems to center in the remarkable phenomena of the Permian 

 period, for at or about that time the earth was affected by glaciation 

 in India reaching southward slightly across the Tropic of Cancer, 

 in Australia reaching northward to the Tropic of Capricorn, and in 

 South Africa covering a large area in Cape Colony, Natal, Zululand, 

 the Orange State, and Transvaal, while at or about the same time 

 prevailing aridit}^ is indicated by salt and gypsum deposits and asso- 

 ciated Red Beds distributed through extraordinary ranges of latitude 

 and longitude. Apparently at no other period of geological history 

 were the phenomena of glaciation and aridity so extraordinarily man- 

 ifested. At the same time it is to be recognized that substantial 

 evidences of glaciation are found as far back as the early Cambrian 

 or pre-Cambrian times (Norway, China, Year Book No. 3, p. 282, 

 Willis), and evidences of very extensive aridity are exhibited in Silu- 

 rian and earlier periods, while both these phenomena have had marked 

 expressions in more recent geological times. On the other hand, 

 very mild climates in polar latitudes are evidenced by marine and 

 land life at several periods in the course of geological history falling 

 between the extraordinary expressions of glaciation. The climatic 

 problem, therefore, instead of shaping itself, as we once supposed, 

 as a question of decadence from an extremely hot, moist, uniform 

 state in the earlier history, to a state of relative aridity and glaciation 



