The Monteregian Hills. 241 



The Structure of Mount Johnson. — The structure of 

 the mountain and the character of the rocks composing 

 it also throw some light on the question as to where the 

 differentiation took place. In course of conversation 

 with the foreman of one of the quarries in the essexite on 

 the flank of the mountain, the writer was informed by 

 him that Mount Johnson consisted of three layers of 

 horizontal rock ; a fine-grained one on top, below which 

 was the coarser-grained rock of the quarry, and beneath 

 this a spotted variety. Each of these layers, he con- 

 sidered, went through the monntain horizontally and 

 could be seen outcropping at their respective levels on 

 every side. The three rocks referred to were, as will be 

 recognized, the fine-grained essexose, the andose, and the 

 transitional rock below the latter, respectively. The 

 pulaskite zone he had not noticed, it being at the base 

 of the mountain and in many places more or less covered 

 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, for- 

 merly 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 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 cir- 

 cular perforation in the horizontal strata of the plane, 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 arrangement is seen in most large exposures of the 

 essexite and with especial distinctness in the great faces, 



