THE CAMBRIAN PERIOD. 



267 



nished the conditions under which the erosion and the deposition took 

 place are less certain, and have been assigned to different sources by 

 different investigators. The preponderant tendency has been to give 

 a leading place to crust al movement. On the other hand, in the previ- 

 ous discussion a preference has been indicated for assigning the leading 

 place to gradation, and for attributing such deformations as obviously 

 took place to superficial causes that were largely inherited from the 

 Proterozoic or post-Proterozoic diastrophism. On this view, the 

 period was one of general quiescence and almost free from profound 

 deformation. This interpretation makes the Cambrian a period of 

 progressive base-leveling, modified in some degree by superficial de- 

 formations. The gradational interpretation has the merit of accom- 

 modating automatically, as it were, the erosion that furnished the 

 material, to the planation that provided the nearly level expanse on 

 which the broad sheet of the later Cambrian was spread. 



Changes in the Cambrian sediments since their deposition. — Since 

 their deposition, the sediments of the Cambrian system have undergone 



Fig. 111. — A section showing the relations of the Cambrian at one point (near Tintic) 

 in Utah. €, Cambrian; C, Carboniferous; frh, rhyolite; an, andesite; Pal, 

 Pleistocene formations. Length of the section, about 6 miles. (Emmons, Tintic 

 (Utah) folio, U. S. Geol. Surv.) 



more or less change. In most regions the gravels, sands, and muds 

 have been compacted and cemented into conglomerates and sandstones 

 and shales respectively. In some places the cementation of the sand- 



Fig. 112. — Section showing relations of the Cambrian at a point in Montana. Ai, 

 Archean; A, Proterozoic; €, Cambrian; C, Carboniferous; D, Devonian. Length 

 of section, about 20 miles. (Weed, Little Belt (Mont.) folio, U. S. Geol. Surv.) 



stone has been carried so far as to convert it into quartzite, though 

 this is not the rule. Over great areas in the interior (Missouri, Wis- 

 consin, Texas, etc.) the strata still remain in horizontal or nearly hori- 

 zontal position (Figs. 102 and 107), while in other regions they have 

 been tilted and even folded. Where close folding has taken place, the 



