CHAPTER XIII 



The Cambrian rocks. Most of the rocks which consti- 

 tute the Cambrian system in the United States were origi- 

 nally sands, clays, and oozes, deposited in nearly horizontal 

 layers upon the bottom of the seas of the Cambrian time. 

 That portion of the deposits from which the sea has since 

 been withdrawn and which has been exposed to view by the 

 removal of such younger strata as were deposited on them, 

 was laid down chiefly in the shallow waters near shores. For 

 this reason the clastic sediments predominate in the Cambrian 

 system as we know it. Embedded in these sediments we find 

 the shells of some of the animals which lived in the same 

 seas. The fossils in the lower layers differ somewhat from 

 those found in the upper beds of the system, and by the 

 gradual changes in the fossils from level to level, several 

 stages, or horizons, have been recognized within the Cambrian 

 system. A threefold division of the system is usually made, 

 giving us Lower, Middle, and Upper Cambrian series, corre- 

 sponding to similar epochs of time. 



Basal unconformity. The lowest layers of the Cambrian 

 sediments generally rest upon an uneven eroded surface of the 

 older rocks. In some places the underlying strata are of 

 Proterozoic age ; in others of Archaean age. Some of the older 

 rocks were folded or even metamorphosed before the Cam- 

 brian strata were laid down. As evidence of this, it is common 

 to find, in the lowest Cambrian beds, pebbles which are water- 

 worn fragments of the older rocks. The unconformity thus 

 indicated has been observed in many parts of the continent, 

 and, as very few exceptions have been discovered, it is evi- 

 dent that before the Cambrian period began, most of North 

 America had been for a time dry land and subjected to ero- 



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