212 The American Geologist. October lom. 
perthite and plai^ioclase becomes a noticeable constituent. As 
in the essexyte a little nepheline, usually quite decomposed, 
and a blue sodalitc appear. The latter occurs in strings 
amono'st the feldspars and like the nepheline was one of the 
latest minerals to crystallize. Some contact features of the 
pulaskyte have already been mentioned and, like those of the 
other two rocks, indicate an increased basicity in the peri- 
pheral portions of the mass. 
Later Dykes. 
Besides dykes of the various classes of rocks which con- 
stitute the main mass of the mountains there are many of later 
age which are themselves of at least two different ages of in- 
trusion. They were distinguished in the field as the light col- 
ored and the dark colored dykes, a distinction which detailed 
study proved to be a natural one. The dark colored dykes 
were found to be intersected in numerous instances by the 
light colored ones and no case of the reverse has been found. 
Hence they are undoubtedly the older. The later or trachytic 
class become in a few cases quite free from bisilicates, have 
the trachytic structure less pronounced, and so pass into bos- 
tonyte, while the earlier series are chiefly laniprophyres. Some 
are of the camptonyte type, while others occasionally assume 
a more plutonic character and by the addition of nepheline in 
essential amounts pass into a hypabyssal form of theralyte. 
Magiiiafic Relations. 
The complementary nature of the dykes as well as of the 
rocks of the first and second irruptions seem tO' indicate beyond 
question that the igneous rocks of Shefford are only slightly 
differentiated products of a single magma. The splitting up 
of an alkaline magma into dykes of the camptonyte and bos- 
tonyte series is too well known to require comment, and in the 
absence of chemical analyses it can only be said that their 
presence indicates an original magma of medium basicity. 
But in the case of greater and consequently the much more 
important masses, it is interesting to note that the pulaskyte, 
the latest of the three intrusions, stands as an almost exact 
mean in chemical composition between the essexyte and 
nordmarkyte of the first and second irruptions. Hence lay- 
