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POPULAR SCIENCE REVIEW. 
at once struck with the very different appearance of these com- 
pared with those just described. Take, for instance, a thin slice 
of a sandstone, no matter how fine-grained it may be ; it is seen 
to be an aggregate of particles, more or less water-worn, of 
quartz and other minerals, evidently derived from the breaking 
up of older rocks. Clays, shales, and most of the slate rocks 
present a somewhat similar appearance, although with a more 
minute structure, which in the slatey rocks is sometimes con- 
siderably affected by the effects of pressure, as well as by other 
agencies since their first deposition. The microscope, again, 
will often bring to view in these rocks very numerous remains 
of organisms. Some of the clays and shales and limestones 
contain foraminifera, diatomacese, and other fossil traces of the 
life of the period. Occasionally rocks are found, such as some 
of the foreign slates and clays, that are composed of little else 
than diatomacese and sponge spicules. The tripoli and semi-opal 
of Bilin, in Bohemia, present fine examples of such rocks. 
Chalk, too, when subjected to microscopic examination, reveals 
instantly its purely organic origin ; it is seen to be built up 
almost entirely of minute organic bodies — foraminifera, sponge 
spicules, fragments of bryozoa, &c. So bog iron ore can be 
traced to a similar origin. The older limestones, also, even 
when no fossils appear to be present, are seen, when sufficiently 
thin sections are prepared, to be similarly composed ; and the 
microscope thus helps us to correlate the limestone and the 
chalk of former ages with formations of a like nature, accumu- 
lating at the present day in the bed of the ocean. 
To the microscope we owe the discovery of what is possibly 
the earliest existing trace of organic life on our earth ; the 
eozoon canadense, which is, if the opinions of those most com- 
petent to judge be accepted, the remains of a gigantic forami- 
nifer, entombed in the laurentian serpentine. 
It is, perhaps, hardly necessary to observe how the microscope 
will also enable the skilled observer to determine from a small 
fragment of bone or of a tooth the nature of the animal to which 
it belonged ; or, once more, in the faint traces of vegetable 
origin frequently met with in certain rocks, to gather some idea 
of the plant life of the period. We all know how abundantly 
a microscopic examination of coal and of the carboniferous vege- 
tation has repaid the labour spent upon it, enabling us to form 
tolerably correct notions as to the nature of the vast flora which 
clothed a great part of the earth’s surface in bygone ages. 
With these scanty and, I fear, very imperfect notes upon a 
wide and most interesting field of study, I bring my paper to a 
close, and I trust that it will not be very long before we find 
many English geologists awaking to the great importance of 
this branch of their science. That it will well repay the most 
earnest attention there can be little doubt. To the petrologist, 
