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POPULAR SCIENCE REVIEW. 
of geologists ; and Mr. Sorbv, in liis Presidential Address to the Royal 
Microscopical Society on March 7, 1877, has shown that similar examina- 
tions with the microscope of the constituents of sands, muds, and clays, 
may lead to exceedingly valuable results. The following is a slight abridg- 
ment of an abstract of his address published by Mr. Sorby in u Nature ” of 
Peb. 22, 1877. The paper appears in full in the u Monthly Microscopical 
Journal.” 
“ The scope of this subject, as treated by the author, included the identi- 
fication of the true mineral nature of the various particles, and the deter- 
mination of the nature of the rock from which they were originally derived ; 
the chief aim being to trace back the history of the material to the furthest 
possible extent. 
11 After describing the manner in which the different kinds of deposits 
should be prepared, examined, and mounted as permanent objects, the 
author indicated the conditions necessary for satisfactorily seeing the various 
particles, and for observing their microscopic structure and optical charac- 
ters. The particles of clay and the fluid-cavities in grains of sand are often 
so minute as to task the power of the microscope to the fullest extent, and 
some indeed are so small that their perfect definition may perhaps be im- 
possible by any means at our command. The conditions under which many 
of the objects are visible are such that with highly convergent light and 
object-glasses of large aperture no dark outline is possible, and therefore they 
are quite invisible, but become quite distinct when the aperture is reduced 
to a moderate and appropriate amount. For this reason object-glasses of 
comparatively small aperture are far the best, since the focal point being 
further from the front lens, very high powers can be used in cases which 
are beyond the reach of lenses of large aperture. 
“ The author then went into much detail to show the character of the 
grains of quartz, mica, and other minerals derived from the decomposition 
of various crystalline rocks, and showed that on the whole there are many 
characteristic differences between the material derived from granitic and 
schistose rocks, consisting mainly in the form, internal structure, and 
optical characters of the constituent grains ; the general conclusion being 
that a careful study of sands, muds, and clays enables us to form a very 
satisfactory opinion as to whether they were derived mainly from granitic 
or schistose rocks, or from a mixture of the two in some approximately 
definite proportion. It was also shown that the shape of the particles as 
originally derived from their parent rock is sufficiently definite and charac- 
teristic to enable us to form a very good opinion respecting the amount of 
subsequent mechanical or other change. 
“ Applying these principles to the study of particular cases, it was shown 
that the coarser British sandstones have been mainly derived from granite 
rocks somewhat intermediate in character between those of the Scotch High- 
lands and Scandinavia. Some of them consist of grains which have undergone 
scarcely any wearing, and are as angular as those derived directly from de- 
composed granite, unlike the blown sands of the deserts, which are worn 
into perfectly rounded grains. 
u The finer sands are no less angular than the coarse. They are not derived 
from the wearing down of larger fragments, but have resulted from the 
