76 
with fallen blocks and talus. If this were the true inter- 
pretation of the structure, the mountain would have to be 
considered as the remnant of a laccolite which had been 
intruded between the horizontal Silurian strata and which 
had subsequently been almost entirely removed by peri- 
pheral denudation. This has been shown to be the true 
explanation of the origin of some of the occurrences, 
Andose in quarry on Mount Johnson, showing vertical flow structure. 
formerly supposed to be intrusive stocks, in the western 
portion of the United States, and it was at first considered 
-as a possible explanation of the origin of MountJohnson. 
A careful examination of the mountain, however, shows 
that such an explanation of its origin is untenable, and that 
it is a true neck, due to the filling up of a nearly circular 
perforation in horizontal strata by an upward moving 
magma. 
