355 
THE MICROSCOPE IN OEOLOOY. 
By DAVID FORBES, F.R.S. 
T HE more searching and exact method of investigation now 
demanded by the advancing state of geological enquiry, neces- 
sitates that the student of that science shall in his researches 
avail himself of all possible means which the collateral sciences 
place at his disposal, and, amongst others, of those which can 
enable him to extend his powers of observation beyond the 
limits to which his unassisted eyesight can convey him. 
As long as he encounters in the field only rocks of so coarse or 
simple a structure as to admit of their being resolved by the naked 
eye into their constituent mineral species, or of distinguishing 
the fragments of previously existing rocks, of which they may 
have been built up, he may speculate with a fair chance of suc- 
cess as to their probable origin or mode of formation. When, 
however, as is more often the rule than the exception, rocks 
are everywhere met with presenting so fine-grained and appa- 
rently homogeneous a texture as to defy such attempts at ocular 
analysis, all speculations as to their nature and formation based 
merely upon observation in the field, can but be compared to 
groping in the dark, with the faint hope of stumbling upon the 
truth. 
In these cases the geologist must call in the aid of chemistry 
and the microscope ; by chemical analysis he learns the per- 
centage composition of the rock in question, and the microscopic 
examination informs him how the chemical elements are mine- 
ralogically combined, and at the same time affords valuable 
information as to the physical structure and arrangement of the 
components of the rock mass, tending to elucidate its formation 
and origin. 
The microscope, employed some thirty years back by Ehren- 
berg in the examination of minute fossil organisms, is now 
recognised as having already done good service to palaeontology, 
but was quite unknown to the geologist proper until within the 
last few years, when the admirable labours of Sorby have 
demonstrated the importance of the microscope as an indispen- 
VOL. VI. — NO. XXV. D D 
