1888.] 511 [Crosby. 



Hills became dry land and continued so during the older Tertiary 

 or Eocene period. The advent of the broad shallow lake of the 

 Miocene or White River Tertiary period is marked by a thin but 

 very persistent bed of coarse loose conglomerate or gravel, which 

 rests on the Cretaceous unconformably at some points. The peb- 

 bles and bowlders of this conglomerate have been derived chiefly 

 from the Archaean quartzites and schists ; and I also found in the 

 Bad Lands of the White River, fully forty miles from the Hills, 

 fragments of rose quartz and feldspar from the coarse granite of 

 the Harney Range. 



Although it is impossible, for the reasons indicated in the pre- 

 ceding paragraphs, to follow Newton in ascribing the uplift of 

 the Hills wholly to the interval between the Cretaceous and Mio- 

 cene, it seems highly probable that the main part of the differen- 

 tial movement, which is evidenced by the tilting and plication of 

 the strata on the borders of the Hills, took place at this time. The 

 canon-like character of most of the drainage channels radiating 

 from the central area of the hills also indicates that the present 

 relative levels of the hills and the surrounding plains have not 

 been maintained for any considerable period of geological time. 

 The idea that the inception of the Black Hills uplift antedates the 

 Cretaceous period was first suggested to me by Prof. Carpenter ; 

 and I am unable now clearly to distinguish my own observations 

 and reflections on this subject from those for which I am indebted 

 to him. 



VOLCANIC PHENOMENA. 



Intrusive Sheets and Laccolites. 

 The structural relations of the volcanic masses which dot the 

 northern end of the Black Hills were as accurately determined by 

 Newton as was, perhaps, possible under the circumstances of his 

 survey and especially before the minds of geologists had grasped 

 the idea of the laccolite. He saw that these masses are not vol- 

 canic in the ordinary sense of the word, that there are no surface 

 flows, no scoriaceous, glassy or fragmental forms of lava, and that 

 the cones are perfectly structureless masses, agreeing onty in ex- 

 ternal form with the products of crater eruptions. He found the 

 various stratified formations, from the Potsdam to the Cretaceous, 

 dipping away from the eruptive masses in all directions, forming 

 concentric circular outcrops about their bases and often exhibiting 



