334 



HISTORICAL GEOLOGY 



Shifting of volcanic activity. In striking contrast to the 

 Keweenawan system, the Cambrian rocks of North America 

 contain scarcely a trace of volcanic materials. As later 

 periods are studied it will be seen that volcanic activity is 

 prevalent in one region for a time and then dies out, only to 

 break forth again in some other district. So in the Cambrian 

 period, Wales and Scotland, to-day entirely without volcanic 

 activity, were the scenes of many eruptions. 



Later changes in the Cambrian rocks. The sediments of 

 which we have sketched the origin have since been changed 



in various ways. 

 Almost all have 

 been converted into 

 firm rocks : the ooze 

 into limestones, the 

 muds into shales, 

 and the sands into 

 sandstones or even 

 quartzites. Along 

 both the Atlantic 

 and Pacific coasts 

 they have been in 

 part metamor- 

 phosed into slates, schists, and gneisses by exceptional com- 

 pression and at the same time their fossils were obliterated. 

 Wherever the sea is, there sediments are being deposited ; 

 and to these must be added the debris laid down in lakes and 

 other low places. The rocks of any period therefore originally 

 formed a layer somewhat more extensive than the seas of their 

 time. Most of that blanket of rock which we call the Cambrian 

 system is still beneath the sea or, if raised above it, remains 

 concealed by the formations afterwards laid upon it. Around 

 the borders of the old Cambrian lands the system now out- 

 crops in an irregular band adjacent to the older rocks, and, in 

 certain mountain regions both east and west, the Cambrian 

 has been exposed by the deep erosion of raised or folded tracts. 



FIG. 329. Block diagram of a dome fold like that 

 of the Black Hills of South Dakota, showing the 

 relation of the Cambrian (solid black) and later 

 sedimentary rocks to the highly folded rocks of 

 pre-Cambrian age. 



