1^0 MINERALOGY AND LITIIOLOGY. 



magnetite, pyrite, and sphene. The rocks are therefore alHed 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. 



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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 



