SUMMAEY OF THE HISTOEY. 221 



an interval during which some erosion and metamorphism took place, the 

 newer Archaean was deposited. Its constitution was about the same, but 

 it was characterized by a greater amount of sandstone. At about the same 

 time a large quantity of igneous rock was introduced between layers of the 

 older Archaean, and there solidified, taking the form of granite. The record 

 is then blank for a period of great duration, and it is only known that at 

 some time during that period the Archaean strata were altered to the condi- 

 tion of crystalline schists and quartzite, and were subjected to a vast dis- 

 placement and erosion, whereby their worn edges were left in a vertical 

 position over the whole area of their present exposure. In this interval 

 there may have been many alternating epochs of sedimentation and erosion. 

 We only know that the closing epoch was one of erosion, and that the local- 

 ity of the Hills was part of a large tract of dry land. 



The next chapter is one of sedimentation. The ocean advanced slowly 

 over the sloping land, sorting and rearranging with its waves and currents 

 the debris that lay upon its surface, and forming the Potsdam conglomerates 

 and sandstones. For a time a ridge of the Archaean seems to have formed 

 an island in the Potsdam ocean, where the Hills now stand, but it was 

 eventually overwhelmed and covered by the shifting sand. After this first 

 advance the waters seemed to have retired for a while, for the Black Hills 

 show no trace of great formations that were accumulated elsewhere ; but 

 there was no local displacement and no deep-cutting erosion, and the sedi- 

 ment of the returning ocean was spread in apparent conformity over that 

 which marks the Potsdam invasion. The new flood was deeper than the 

 old, and it deposited at first only limestones, but after a time it became 

 shallower, and in the latter part of the Carboniferous period many beds of 

 sand were interpolated among the calcareous deposits. 



Up to this time the ocean was full of life, but now the conditions were 

 changed in some way, and life seems to have been banished. Perhaps 

 there was a glacial epoch, and all forms that could not migrate perished ; 

 or perhaps the local waters were by some revolution cut off from the gen- 

 eral ocean, and, like the Dead Sea, became saturated with a poisonous min- 

 eral. When life at last returned the sea was so shallow that the (Jurassic) 

 deposits were ripple-marked, but there was no exposure of land surface. 



