OF DENISON UNIVERSITY. 
7 
local rock to a plastic mass, which upon recrystallization assumed the 
distinctive characteristics noted above. 
The next rock studied from this neighborhood is a piece of fine- 
grained gneiss, of dark geenish gray color almost schistose structure and 
is from the rock surrounding the trap. When studied microscopically, 
we find the greater part of it to be hornblende in short, irregular crys- 
tals, with a few large crystals of the same containing inclusions of apa- 
tite, with magnetite and calcite as product of alteration. Quartz is 
present with a few scales of biotite and many crystals of apatite. One 
thing noticeable in the rock, is the peculiarity of the form and occur- 
rences of the calcite, In almost every case observed, it was formed 
either lying within or adjacent to a crystal of hornblende and always 
enclosing grains or skeletons of magnetite, except in two or three cases 
and in those cases the enclosed particles are hematite, which are un- 
doubtedly alteration products of magnetite. The calcite has apparent- 
ly assumed the form of the original magnetite crystal or the space oc- 
cupied by it and the adjacent hornblende alteration product. 
Fig. I. Sec. A., is a crystal of hornblende containing calcite (c) undoubted- 
ly an alteration product, and enclosing a crystal of magnetite; (a) is a cross sec- 
tion of apatite. Fig. 2, is a crystal of calcite with slightly greenish tinge, as in- 
dicating its probable origin, showing the cleavage of calcite and surrounding a 
grain or skeleton of magnetite. Fig. 3, is a crystal of calcite vdth an enclosed 
scale of hematite altered from magnetite. It will be noticed that the calcite 
grains are nearly regular in shape and apparently assume the form originally held 
by the magnetite. Such is the case in almost every instance. There are how- 
sever, a few crystals, as in Fig. 4, where the hornblende has altered along the 
«dges with the magnetite skeleton within, and the whole forming an irregular 
crystal. 
Two theories are proposed in regard to this change; one is that it 
was brought about by the presence of carbonic acid in the interstitial 
water of the rock,' by means of which interaction was set up between 
the magnetite and hornblende, substituting iron for calcium in the lat- 
ter, depositing some of its calcium as calcite about the remnant of the 
magnetite ; or it may be a pseudomorphic product of the hornblende 
leaving the magnetite as a residue. 
Our two next studies are different phases of granite from the 
quarry about two miles east of the dyke. The rock is a true granite 
and is extensively quarried. Macroscopically they differ in general 
only in color. One a dark and the other a light gray, with, perhaps, 
slight indications of weathering on the surface of the darker rock. It 
