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ON THE MICROSCOPICAL STRUCTURE OF MOUNT SORREL SYENITE, 
ARTIFICIALLY FUSED AND COOLED SLOWLY. BY H. C. 
SORBY, ESQ., F.R.S, F.G.S., &C, OF SHEFFIELD. 
My object in bringing forward this subject is more to 
exhibit the specimens themselves than to give any thorough 
description of their structure. This would be very difficult, 
except aided by numerous careful drawings, and would even 
then fall far short of what can be seen at once by inspecting 
the thin sections with the microscope. 
I am indebted to Mr. James G. Marshall, F.G.S., of 
Headingley House, Leeds, for the material used in my 
microscopical inquiries. He melted in some cases above a 
ton of the rock, and allowed it to cool very slowly. I have 
prepared thin sections of various specimens, and I may 
remark that the facts I am about to describe form part 
of a wider inquiry in which I have for some time been 
engaged, and that I have already studied, and shall still 
further most carefully study, the comparative structure of 
various kiuds of igneous rocks in their natural state, 
and after having been fused and slowly cooled. However, 
not to take up time with such general remarks, I will at 
once proceed to describe the structure of the Mount Sorrel 
Syenite, and of the products derived from it by fusion. 
The rock operated on is a mixture of reddish felspar, clear 
green hornblende, and quartz, along with some opaque 
mineral, evidently in a greatly altered state, perhaps 
originally pyrites or magnetic oxide of iron. The felspar 
is in very distinct crystals, but has often caught up much 
hornblende ; and the quartz fills up the spaces between the 
other minerals, or is curiously crystallized along with the 
felspar, so as to form a microscopic " graphic granite," or 
"hebraic felspar;" and it is especially important to bear 
in mind that the quartz contains very many fluid-cavities, 
