THE MONTEREGIAN HILLS 277 



Dykes. — A feature in connection with Mount Johnson, and 

 one possibly connected with its somewhat peculiar structure, is 

 the almost entire absence of dykes. These were found only in 

 two places, and in both cases the dykes were small in size. The 

 first of these localities is on the northeastern margin of the 

 intrusion, where the dyke occurs in association with and prob- 

 ably cutting the hornstone. It was found as large angular 

 blocks in the heavy maple bush which here covers the slope of 

 the mountain, but is undoubtedly in place in the immediate 

 vicinity. The rock is very dark gray in color and very fine in 

 grain, and belongs to the camptonites. It has a porphyritic 

 structure, the very numerous phenocrysts consisting of horn- 

 blende and pyroxene. The hornblende phenocrysts are deep- 

 brown in color and strongly pleochroic, the mineral being the 

 same basaltic hornblende described in the essexite. The pyrox- 

 ene of the phenocrysts is pale purplish in color and shows a 

 marked dispersion of the bisectrices. Both minerajs have very 

 perfect crystalline forms. The plagioclase of the rock is very 

 basic in character, as shown by its high extension. The rock 

 resembles very closely certain occurrences found on Mount 

 Royal. The size of this dyke is not known, but it probably has 

 not a width of more than a foot or two. The other dykes occur 

 on the southeastern slope of the mountain by the side of the 

 road leading down from the quarries here. At this locality there 

 are four small dykes, the largest only a foot in width, cutting the 

 essexite. These are all very fine in grain and much decomposed, 

 but represent two varieties of rock. Two of the smallest are 

 composed of a camptonite consisting of a groundmass of brown- 

 ish hornblende and plagioclase, with lath-shaped plagioclase 

 phenocrysts. The other two dykes consist of a rusty weathering 

 rock, made up of feldspar laths and a mass of pseudomorphs of 

 limonite after some prismatic mineral, probably either aegerin or 

 arfvedsonite. Professor Rosenbusch considers it to be a highly 

 altered tinguaite or solvsbergite, probably the latter. 



The several dykes, while small and unimportant in them- 

 selves, are of interest in that they present the petrographical 

 types regularly associated with the alkaline rich intrusions of 

 the class represented in Mount Johnson. 



