330 



HISTORICAL GEOLOGY 



QUESTIONS 



1. Pebbles of Archaean schist are of ten found in the basal layers 

 of the Proterozoic rocks. Under what conditions are schists pro- 

 duced? And what does this tell about the depth to which the 

 erosion of the land had penetrated at this time ? 



2. What events are recorded by the unconformities in Figures 

 326, 327, and 328 ? 



FIG. 326. An irregular 

 contact between hori- 

 zontal beds. 



FIG. 327. Horizontal Fu. 328. Horizontal 

 sandstone resting up- sandstone resting up- 

 on folded beds. on granite, schist, and 

 slate. 



3. With which type of volcanic eruption are cinders and ashes 

 usually associated, the fissure or the crater type ? 



4. Why should the gravel beds and cindery layers of the Ke- 

 weenawan contain richer copper ores than the dense lava flows? 



5. Why should the older Proterozoic formations be more folded 

 and metamorphosed on the average than the younger? 



6. Beds of conglomerate thought to be of glacial origin have 

 recently been found in the Huronian rocks of Canada. With which 

 theory of the origin of the earth is this more harmonious ? 



