244 IRVING. 



ified his views very much had he been able to see the eruptive 

 region of Terry peak is unquestionable. It is further not to be 

 doubted that the fluidity of a magma has a great influence on 

 the amount of lateral expansion, but that seems to have been a 

 minor factor in all of the Black Hills intrusions. 



Let us now briefly consider the other causes. It is at once 

 obvious that if the amount of fluid rock is large, other consid- 

 erations being equal, there will be a greater tendency to dome 

 the overlying beds than with a small mass, which may easily 

 spread out along horizontal strata. If we then consider the 

 violence with which the fluid is injected this will be the more evi- 

 dent. With a great force slowly applied, and acting through a 

 long period of time, opportunity for lateral expansion, will be 

 afforded even to a large amount of fluid and long, thin sheets 

 will result. 



If, on the other hand, the force be violent and rapidly applied, 

 however fluid the magma may be, its amount will be greater 

 than can expand in the given time and a doming of strata or 

 even a rupture will result. 



Most important of all, however, are the influences of the 

 rocks into which the magmas are intruded ; and in the areas 

 which the writer has studied, this has been almost without ex- 

 ception the determining factor of the form assumed by the in- 

 trusion. 



2. PETROGRAPHY OF ERUPTIVE ROCKS. 



A. Summary. 



The crystalline rocks of the Black Hills were studied by Cas- 

 well and described by him in the report made by the Newton 

 Survey in 1874. Considering the very elementary condition of 

 petrographic knowledge at the time this work appeared, the de- 

 scriptions are extremely accurate, and show unusually pains- 

 taking labor. Since that time, however, studies of soda-rich 

 rocks in other localities have made possible a much more ac- 

 curate determination of phonolitic rocks than was then possible. 



Caswell's report and rock determinations made by Pirrson 



