170 MINERALOGY AND LITHOLOGY. 
magnetite, pyrite, and sphene. The rocks are therefore allied to diorites. 
The interspaces in another section are filled with an entirely different 
substance, which probably is a very impure pyroxenic mineral, too 
opaque, even in the thinnest sections, for determination, Dr. Hunt told 
me that these rocks were about identical with his norites; and, as the 
microscopic examination presents no objection to this, it cannot be said 
that these rocks were not brought from the great Norian formations in 
Canada described by Dr. Hunt. 
* * * * * * * * * 
Remarks concerning basic eruptive rocks. Looking backward now at 
the general nature of our basic eruptive rocks, there are a few conclu- 
sions to be drawn which are not only instructive in reference to them, 
but are also helpful in our study upon the classes of rocks which are 
hereafter to be considered. 
It is to be noted that there are wide differences in them, which are 
of three kinds. The first results from a difference in the composition of 
the original mass; the second is a difference in structure; and the third 
is a difference due to alteration and decay. We will briefly consider 
these differences and their causes. 
First: in regard to difference in original composition. Eruptive rocks 
are derived from those layers of fused and liquid rock materials which 
underlie the earth’s cold crust. It was long supposed that the whole 
earth was molten and fluid, with the exception of the crust, and that rifts 
in this crust gave passage to the molten materials that were beneath it, 
and which had never been solidified. The demonstration by Hopkins, 
that solidification induced by pressure began at the centre, and that the 
unsolidified zone of the earth is one which lies between the core solidified 
by pressure and the crust solidified by cooling, introduced some new feat- 
ures, since, as shown by Scrope, portions of the earth once solidified 
might become again fluid on account of movements in the earth’s crust 
and the transportal of sediments, resulting in the derangement of the 
balance between the two elements which determine the position of the 
fluid zone. Scrope, Scheerer, Elie De Beaumont, and others have held 
it to be a fundamental circumstance, that water, when present in small 
amount in the materials of rocks which are subjected to heat and pres- 
sure, causes them to become plastic at a temperature far below the point 
