THE CAMBRIAN PERIOD 



407 



equal time in more favorable locations. This should be kept in mind 

 in future discussions, since too often the student forgets that a com- 

 paratively thin formation may have required in its upbuilding as long 

 or a longer time than a much thicker one of different material. 



Present Condition of the Sediments. — Some of the Cambrian sedi- 

 ments have undergone important changes since their deposition. The 



Fig. 376. — Section showing the relation of the Cambrian, Cs, and overlying strata to 

 the Pre-Cambrian gneiss, gn. Crested Butte, Colorado. 



gravels have been changed to hard conglomerates ; the sands to sand- 

 stones, and where the quartz grains have been cemented by quartz, 

 into flint-like quartzites ; the calcareous ooze of the clear seas into 

 limestone. When metamorphism has been intense, shales have been 

 converted into slates and schists, sandstones into schists, and lime- 

 stones into marble. All of the Cambrian formations, however, have 

 not been metamorphosed, some having been little changed. In many 

 places the Cambrian strata have been intensely folded, tilted, and 

 faulted (Fig. 376). Some of the mountain ridges of the Appalachians 

 are formed of the hard, upturned edges of the quartzites of this age. 

 In other regions, as in Wisconsin and northern Minnesota (Fig. 377), 



DRIFT 



Fig. 377. — A section in northern Minnesota, showing the relation of the Cambrian 

 to the Pre-Cambrian strata. 



where the comparatively thin beds are not folded, the formations 

 spread over a wide extent of territory. 



Volcanism. — The Cambrian seems to have been a time of little 

 volcanic activity over the greater part of the world. In North 

 America scarcely a trace of volcanic material has been discovered. 

 Scotland and Wales, however, were the scenes of intense volcanic 

 activity. 



Close of the Cambrian. — The Cambrian is not separated from 

 the rocks of the overlying system (Ordovician) by great unconformi- 

 ties, although local ones exist, but so gradual was the change that it 



