l:;i I. c. RUSSELL URFACE GEOLOGY OF A.LASKA. 



the influence of existiiiLT climatic conditions on the decomposition of rocks 

 throughout a wide range of latitude. 



i of jirniioiui'-'.'d Rock Droti/ in .l/./.s//. — The slight alteration that 

 the surface ruck- in the uonglaciated region of Alaska have suffered, is shown 

 by their freshness, wherever exposed, and hy the total absence of residual 

 clays like those which form such a conspicuous feature of many portions of 

 temperate and tropical countries. Nowhere in Alaska did I see more than 

 a trace of the red and yellow clays which result from the atmospheric decay 

 of a great variety of rock.-. 



( >n Onalaska island the evidences of a general glaciation are absent, but 

 a great extension of local ice streams took place during the Pleistocene 

 period, and resulted in the removal of much of the previously accumulated 

 superficial debris. The ahseuce of marked alteration in the surface outcrops 

 and the lack of brilliantly colored clays might, therefore, in this instance be 

 accounted for by glacial action. 



My observations were continued at St. Michaels, however, and all the way 

 up the Yukon to the eastern border of the uonglaciated area near the mouth 

 of Big Salmon river, and also for about 200 miles up Porcupine river. 

 Throughout this entire region there is a marked absence of pronounced 

 chemical alteration in the rock surfaces. This statement applies to rocks of 

 many kind-, including limestones, sandstones, granites, and various volcanic 

 locks. Moreover, there is practically an entire absence of residual clays. 

 The colors one sees in the rocks are usually various tone- of gray ami brown. 

 The brilliant colors due to oxidation of iron, SO prevalent in regions of 

 marked subaerial decay, are absent. 



( 'omp orison vilh other Regions. — Rock decay in tropical countries is 

 known to lie -reat, as has been shown in the memoir already referred to. In 

 the southern Appalachians the brilliantly colored residual clays frequently 

 have a depth of more than a hundred feet over great area-. In the driftless 

 area of the upper Mississippi valley, a- shown by < !hamberlin and Salisbury, 

 the residual deposits have an average depth of about seven feet, with a max- 

 imum thickness of possibly ten time.- the average. In the driftless area of 

 Alaska, which extends north of the Arctic circle and probably reaches the 

 Arctic ocean, residual deposits, a- already stated, are absent 



Observations in the United States alone thus extend over fully forty de- 

 of latitude, and prove thai rock decaj is a direct result of existing 

 climatic conditions. The elements of climate which exert the greatest influ- 

 ence on exposed rock surfaces seem to lie temperature and moisture. Rocks 

 decay most rapidly in waii 1 1 regions, where the rainfall is abundant, and are 



ireely ;it all decayed in arid >.r frigid regions. 



, Rep . 1--1 SB, Washington, lit 



