THE NATURAL HISTORY OF IGNEOUS ROCKS. 211 



like domes rising through and above level flows of leucito- 

 phyre and leucitite in the Leucite Hills of Wyoming, 

 recently described by Kemp (17). 



Returning to the laccolite, it may be observed that even 

 in the High Plateaux of the West, as the Rocky Moun- 

 tains are approached, the regular lenticular form of intrusion 

 gives place to what has been termed an asymmetric laccolite. 

 This is seen in Mount Marcellina and other examples in 

 the West Elk Mountains of Colorado. The intrusion of 

 Mount Axtell is described as " a sheet greatly thickened at 

 one extremity". In some cases such a form is seen to 

 stand in direct relation with a sharp monoclinal fold, which 

 may pass into a fault in an upward direction. Here it is 

 no longer possible to regard the disturbance of the strata 

 as the direct result of the igneous intrusion, but we may 

 perhaps consider the folding and the intrusion as closely 

 connected results of the same set of forces in the earth's 

 crust. In the Rocky Mountain belt itself, for instance in 

 the Leadville district of Colorado, as described by Bulkley 

 (18), we go a step farther. The Palaeozoic strata are here 

 thrown into large folds, in which occur intrusive bodies of 

 rock comparably petrographically with the typical laccolites 

 of the High Plateaux to the west. To the lateral thrust 

 which has affected the strata must be ascribed not only the 

 folding, but also the formation locally of gaping cavities 

 along the bedding-planes, into which the igneous magma 

 has been forced. It is not probable that actual vacant 

 spaces were ever formed, the inflow of the magma being 

 presumably concurrent with the process of folding. If, 

 however, we are to separate -these two results of a common 

 cause, we must regard the disturbance of the strata as the 

 occasion of the intrusion, not as its consequence. 



From the point of view just indicated we perceive an 

 essential distinction between the original laccolites of Gil- 

 bert and the lenticular intrusive bodies in folded regions to 

 which the title has been extended by some writers. With 

 reference to the disturbance of the associated strata, the one 

 type is an antecedent, the other a consequent phenomenon. 

 A distinctive name for the latter type is still a desideratum. 



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