152 MELLO: MICllOSCOPICAL STRUCTURE OF ROCKS. 
detect the differences existing between similar-looking rocks from 
different localities, or belonging to different periods. Again, with 
regard to the organic remains in rocks, whilst we can readily see the 
larger fossils, there are myriads of smaller ones which require the 
microscope for their detection. Some of the Eocene Limestones, for 
instance, consist of little besides minute foraminifera, as many as 
58,000 of them, it is said, are contained in every cubic inch ; the 
Silurian and Carboniferous, as well as other limestones, when micro- 
scopically examined, arc found to contain myriads upon myriads of 
organisms too small to be seen by the naked eye. 
There are other rocks, such as the Tripoli and semi-opal of Bilin, 
in Bohemia, which are almost entirely composed of diatomacere, and 
spicules of siHcious sponges. Such spicules, together with foramini- 
fera, not only form the bulk of chalk, but also crowd the nodules of 
flint so common in that formation. DiatomacccC also enter largely 
into the composition of bog iron ore In the examination of the larger 
fossils the microscope is often of great use in working out details of 
structure. We know^ too, how nnich has been done by its means in 
determining the structure of fossil plants : for instance, in proving 
the vegetable origin ?o.' many varieties of which are found to be 
entirely built up of minute parts of plants. It is however more in 
connection with the mineral history of the rocks that the vast 
importance of microscopical examination will be seen. 
Chemistry can tell us the elements of which any given mineral 
is composed, and the proportions in which those elements exist in it. 
So can that science also tell us the composition of any rock; but in 
the case of the rock, all that chemistry can do is to say what elements 
enter into its formation, and the quantities in which these occur; un- 
less the separate minerals of which the rock is built up can be clearly 
seen, it cannot tell us how they are distributed in it. Nor can the 
chemist give us any decided clue as to the origin of the rock in 
question, save that he can sometimes gather from the presence of 
carbonate of lime, or carbon and other elements, that organic life had 
probably some part in the formation of the rocks in which these are 
found. But neither chemistry nor the unaided sight can discover 
what part igneous or aqueous actions have played in the history of a 
