220 GEOLOGY OF THE BLACK HILLS. 



strata of the Tertiary nowhere lie in such proximity to the igneous peaks 

 that their structural relations could be made out. All, therefore, that we 

 can learn from the relation of the exotic masses to the strata is that the 

 epoch of their introduction was later than the Fort Benton Cretaceous. 



The opposite limit is shown by the contents of the Miocene conglom- 

 erate. At the base of the White River deposit Mr. Jenney found not only 

 granite and quartzite from the Archaean, but pebbles of trachyte, and these 

 could only have been derived from the Black Hills bosses. The lavas were, 

 therefore, introduced, solidified, and at some point subjected to erosion 

 before the beginning of the Miocene. 



The indicated interval comprises the Niobrara, Fort Pierre, Fox Hills, 

 Judith River, and Fort Union epochs ; a period of time represented by a 

 total depth in the upper Missouri region of about 4,000 feet of sediments. 

 The date of igneous activity is, therefore, very far from established, and 

 its relation to the uplift of the Hills is not even determined. The trachytes 

 may have been in place, and even ancient, when the uprising began, or 

 they may have been forced up while the greater movement was in progress 

 and well advanced. 



A study of the drainage here affords no aid. A rising current of mol- 

 ten rock cannot flow with secular slowness through colder rocks on account 

 of the interchange of temperatures and resulting congelation ; and for this 

 reason an uplift of strata by volcanic force is of necessity catastrophic. In 

 all ordinary cases antecedent drainage is thrown back and turned aside, and 

 a local system consequent to the new mountain takes its place. The drain- 

 age of the igneous uplifts of the Hills is essentially, though imperfectly, 

 consequent, but no conclusion can be drawn from the absence of an ante- 

 cedent system. 



5. — Summary of the history of the hills. 



Putting together the salient events recorded by the character and 

 arrangement of the rocks, as well as by the arrangement of the drainage 

 lines, we have the following history : 



The older Archaean was first laid down. It consisted in chief part of 

 shales and with these were some sandstones. Over it, and probably after 



