THE MONTEREGIAN HILLS 279 



nant of a laccolite which had been intruded between the horizon- 

 tal Silurian strata and which had subsequently been almost 

 entirely removed by peripheral denudation. This has been 

 shown to be the true explanation of the origin of some of the 

 occurrences, formerly supposed to be intrusive stocks, in the 

 western portion of the United States, and it was at first consid- 

 ered as a possible explanation of the origin of Mount Johnson. 

 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 the horizontal strata of the plain, by an upward moving 

 magma. 



The evidence of this is to be found in the direction of the 

 banding or fluidal arrangement of the crystals in the essexite 

 already referred to and shown in Fig. 5. This fluidal arrange- 

 ment is seen in most large exposures of the essexite and with 

 especial distinctness in the great faces of this rock exposed in 

 the quarries on the mountain side, and it is alwaysVertical, show- 

 ing that the movement of the rock was upward through the pipe, 

 and not outward and horizontally over the pulaskite, as it would 

 have been in the case of a laccolite. Furthermore, in several 

 cases when the fluidal arrangement is very distinct and has a 

 somewhat banded character, as shown in Fig. 6, due to the 

 alternation of somewhat more feldspathic portions of the rock 

 with others richer in iron-magnesia constituents, a strike can be 

 made out on horizontal surfaces, and this strike curves around 

 the mountain, following its marginal outline, as shown in the 

 map. Fig. 2. 



It is thus clear that Mount Johnson is a neck in its most typi- 

 cal form. A cross-section of the mountain is shown in Fig. 7. 

 The opening occupied by the intrusion was in all probability 

 formed by the perforation of the horizontal shales at this point 

 by the explosive action of the steam and vapors preceding" the 

 eruption proper, as it presents exactly the features reproduced 

 by Daubree in his highly suggestive experiments on the pene- 

 trating action of exploding gases. It is, in fact, what he terms 

 a diatrhric. 



