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ON THE MICROSCOPICAL STRUCTURE OF ROCKS. BY THE REV. J. MAGENS 
MELLO, M.A., F.G.S., ETC. 
Whilst Geology was still in its infancy, more attention was paid 
to the broader or more general features of the earth's structure and 
past history, than to the minuter details of its component parts. The 
arrangement of the rocks forming that portion of its crust accessible 
to man, their organic contents as affording a key to their relative 
Chronology, the physical causes producing the various changes that 
are seen to have taken place in connection with these rocks and their 
enclosed fossils, the chemical composition of the different minerals, 
these were amongst the principal objects set before the Geological 
student, and are those which still largely engage his attention; but 
of late years we have been taught to see that would we thoroughly 
comprehend the history of the rocks of our globe, we must not 
only study them on a large scale, not only analyse them in the 
chemical laboratory, but we must not rest satisfied until we have 
searched into their innermost structure, and have learned to un- 
ravel by means of the microscope and the polariscope the wondrous 
history of their formation, and of the varied changes that they have 
passed through during the lapse of ages. 
There are many questions which cannot be answered by the 
unaided eye. We can, indeed, by the eye alone learn to distinguish 
between most of the ordinarily met with igneous and aqueous rocks. 
A mere tyro in geology sees at a glance the difference between grits, 
sandstones, limestones, shales or slates, and can discriminate between 
them and the commoner igneous rocks, such as granites, dolerites, 
etc. The crystalline or vitreous texture of the igneous rocks will in 
most ca,ses afford a sufficient indication to enable them to be recog- 
nized by the eye alone; but the eye can tell us nothing without 
instrumental aid as to the minute structure of such rocks, nor can it 
read the history of their origin, and of the subsequent changes they 
may have experienced: neither can it, without the microscope. 
