170 GEOLOGY. 



Conglomerate beds which are not basal also occur, and point to changes 

 in the conditions of sedimentation even where unconformities were not 

 developed. 



The Proterozoic group also contains great thicknesses of quartzite, 

 which is composed chiefly of grains, and sometimes small pebbles, 

 of quartz, cemented firmly together, usually by the secondary growths 

 of the grains, as explained and illustrated in Volume I (p. 434). It 

 is inferred therefore that in Proterozoic times there was ample oppor- 

 tunity for the separation of quartz grains from the quartz-bearing 

 Archean rocks, and for the rolling and rounding of these grains before 

 they came to rest. As the quartzites of the Proterozoic are some- 

 times thousands of feet thick, the decomposition which furnished the 

 necessary sand must have been correspondingly great, since in some 

 great formations made of decomposable minerals (such as feldspars, 

 amphiboles, etc.) grains of quartz are absent or rare. 



The Proterozoic formations also include great beds of shales, or 

 their metamorphic equivalents, which are interpreted as the clayey 

 products of the same decomposition, and their presence confirms the 

 inference drawn from the sandstones and quartzites. Limestones 

 also are present, from which it is inferred that the waters of the time 

 had become calcareous by processes similar to those now in operation 

 (Vol. I, pp. 429-431, 122, and 107), and that by one or another of the 

 processes by which limestone is now produced, a portion of the cal- 

 careous content of the waters was deposited. The limestone affords 

 further confirmation of the preceding inferences, for in the decom- 

 position of crystalline rocks necessary to free the quartz grains and 

 produce clays, the calcareous material is largely dissolved and carried 

 to the sea in solution. 



The inference that these ancient sediments were deposited in the 

 same manner as the sediments of modern times is further supported 

 by the ripple and other water-marks on the sandstones and shales, 

 and by the character of their lamination and stratification, which, 

 where still preserved, are precisely analogous to those of the present. 

 By combining evidences of this kind, until by mutual support there is 

 no rational room for doubt, the mode of formation of these ancient 

 deposits is deciphered, and their history read. 



The fact that some of the deposits are differently constituted, and 

 tell a different tale, tends rather to increase confidence than to destroy 



