1 82 STUDIES FOR STUDENTS 



is faster in the warmer or cooler climate. In any case, it is evident 

 from the preceding discussion that a series of climatic oscillations 

 involving merely temperature changes would find record in the varying 

 kind and rate of erosion and consequent sedimentation in regions 

 either where this climatic change was between cold and temperate 

 or between temperate and torrid limits. 



Separation of the Topogeaphic and Climatic Factors 



As previously stated, under the relations of rainfall and topography 

 to erosion, young, mountainous topography not only gives rise to 

 rapid erosion, but accentuates climatic contrasts, so that a marked 

 distinction may still have opportunity to become developed between 

 the products of erosion of humid and arid mountain regions. The 

 .extent to which this is true may be seen by comparing the alluvium 

 of the Rio Grande with that of the Missouri- Mississippi system, where 

 in silts of the same degree of fineness that from the arid region shows 

 a much higher ratio of soluble constituents.^ The researches of the 

 geologists of India indicate the same contrast between the alluvium 

 of the Indo-Gangetic plain and that of the Brahmapootra in southern 

 Assam. ^ In these examples the material is derived from regions of 

 high relief and rapid erosion. Gravels or cobbles may be deposited 

 under such circumstances nearer the sources, but the production of 

 the conglomerate has involved initial rock-breaking and the produc- 

 tion of a considerable quantity of fine material which may occur as a 

 matrix or as separate deposits of clay or silt. These finer materials, 

 as stated, liave distinctive characters in each strongly marked climatic 

 province. 



It is concluded, therefore, that an examination of the character 

 of the matrix or associated fine beds is of importance in determining 

 the climatic conditions attending the origin of a terrestrial conglomerate 

 or sandstone. This conclusion may be illustrated by contrasting 

 the red sandstones and shales, occasionally conglomeratic, of the 

 Connecticut Valley, with the predominantly gray conglomerates 

 and black shales of the Carboniferous basin of Rhode Island; the 

 two regions being separated by less than fifty miles, and both contain- 

 ing sediments of rather local origin. There are strong evidences in 



I Hilgard, Soils, 1906, pp. 368, 378. ^ Hilgard, op. cit., p. 413. 



