THE BEARING OF PROGRESSIVE INCREASE OF 
VISCOSITY DURING INTRUSION ON THE 
FORM OF LACCOLITHS 
SIDNEY PAIGE! 
United States Geological Survey 
The attention of geologists has been called on several occasions 
to the laccolithic intrusive bodies of the northern Black Hills, 
South Dakota. Russell? has described and offered explanations for 
a number of the phenomena there observed. Jaggar and Howe? 
have published a detailed account of the region and performed 
experiments illustrating the processes which are believed to have led 
- to the formation of laccolithic intrusive bodies ; and others have com- 
mented on the work of these men, in discussing other regions where 
similar phenomena may be observed. The classic work of Gilbert4 
and the equally careful studies of Pirsson and Weed,’ Cross,® and 
others have contributed to placing the theory of laccolithic intrusion 
on a firm basis. The type of laccolithic structure about to be 
described has certain peculiarities, to explain which the writer 
resorted to speculation and came to conclusions for which he later 
found partial support in the accounts of earlier writers, particularly 
Pirsson. In the case in point the process which is invoked as an 
* Published with the permission of the Director of the United Stated Geological 
Survey. 
2Tsrael C. Russell, “Igneous Intrusions in the Neighborhood of the Black Hills 
of Dakota,” Jour. Geol., IV (1896), 23-43. 
3T. A. Jaggar, Jr., The Laccoliths of the Black Hills; Ernest Howe, “A Chapter 
on Experiments Illustrating Intrusion and Erosion,” Twenty-first Ann. Rept. U.S. Geol. 
Survey, Pt. 3 (1901), pp. 165-303. 
4G. K. Gilbert, ‘““Geology of the Henry Mountains,’ U.S. Geographical and 
Geological Survey of the Rocky Mountains Region, 1880. 
5W. H. Weed and L. V. Pirsson, ‘‘Geology and Mineral Resources of Judith 
Mountains of Montana,” Eighteenth Ann. Rept. U.S. Geol. Survey, Pt. 3 (1808), 
PP. 445-614. 
6 Whitman Cross, “The Laccolitic Mountain Groups of Colorado, Utah, and 
Arizona,” Fourteenth Ann. Rept. U.S. Geol. Survey, Pt. 2 (1892-93), pp. 165-258. 
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