BLACK HILLS GEOLOGY. 243 



that of the thinly-bedded shales below, has been exceeded ; faults 

 have sometimes taken place, and the intruded mass has lifted up 

 large blocks of sediments and filled the space below them. Ero- 

 sion has then removed the coverings and left us the "plug-" 

 like and laccolitic masses. In the case of Ragged Top, the 

 plug-like aspect seems almost unquestionably due to the massive 

 character of the limestone. With the Needles this is also true. 

 In the case of Crow peak no faulting seems to have occurred, 

 but the intrusion is of the same general type. Sugar Loaf hill 

 is a true laccolite. 



If we bear in mind the influence which the 600 or 800 feet of 

 massive Carboniferous strata have exercised on the rocks in- 

 truded below — first by virtue of their position over a thinly 

 bedded, fissile series of shales, such as the Cambrian, and second 

 by virtue of their massive character — we can more readily un- 

 derstand the unique nature of the intrusions that form the 

 outlying peaks of the Black Hills region. The absence of dikes 

 and small auxiliary intrusions is thus accounted for, because 

 only those intrusions which have been very strong and locally 

 violent have been able to penetrate beyond this heavy formation. 

 Where erosion has removed this series the intrusives are ex- 

 posed in great abundance and probably exist in equal profusion 

 in the Cambrian shales far below the existing exposures of the 

 outlying peaks. 



Causes Influencing the Formation of an Igneous Intrusion. 



From these observations we may classify the causes that have 

 influenced the form of the intrusions as follows : 



r . r 1st. Due to pressure exerted by overlying rocks. 



T , , I ■ ■ -J-, i 2d. Due to chemical composition of the intruded 



Internal -{ viscidity ^ 



j -' (^ magma. 



L B. Volume of magma intruded. 



{C. Lithological character of rocks into which the mass has been in- 

 truded. 

 D. Violence of force of intrusion. 



It is to the first of these causes that Russel attributed the 

 peculiar "plug "-like form of the Black Hills intrusion, but to 

 the other causes he does not refer. That he would have mod- 



