238 IRVING. 



C. VIEWS OF PREVIOUS WRITERS. 



Probably no geological feature of the Black Hills has at- 

 tracted a greater amount of attention than the eruptive rocks. 

 The number of small igneous peaks there exposed is so great, 

 and they occur so closely crowded together within a compara- 

 tively limited area, and show such unique structure, that it is 

 quite natural that, even with the very superficial examinations 

 given to them, they should have become widely known. 



Newton was the first to examine them, and was impressed 

 with the extremely local character of the disturbance which 

 they had produced upon the encompassing sediments. This fact, 

 taken together with the manner in which the sediments were 

 upraised about their sides, led him to account for them by the 

 theory of pustular eruptions. He considered them as eruptive 

 masses that had broken up through the overlying rock, reach- 

 ing up to and extending beyond the surface so as to leave the 

 strata uplifted around them, just as are the broken edges of a 

 piece of paper when it has been penetrated by a pencil. Such 

 an assumption, as Professor Crosby has stated,^ necessitates a 

 degree of viscidity which it is difficult to imagine in any magma 

 that has reached the surface, for in no case has the rock flowed 

 outward from the center of the eruption. 



Some years after Newton's report had been made, Professor 

 Crosby visited the Hills and, in company with Dr. F. R. Car- 

 penter, studied the various formations exposed. 



In a short discussion ^ of the igneous phenomena he calls 

 attention to the occurrence of true laccolites, a form of intrusion 

 unknown at the time of the Newton survey ; and of vast num- 

 bers of sheets and dikes occurring in the hills together with them. 



He then shows that the existence of these thin, conformable, 

 intruded sheets necessitates a degree of fluidity in the rock 

 which is entirely at variance with Newton's theory. 



Finally Professor I. C. Russell visited the region. He did 

 not study the same portion of the country as Crosby, but con- 



1 Proceedings of Boston Society of Natural History, Vol. XXII., page 513. 

 'Op cit. Page 512. 



