102 . The Microscopical Examination Koy 
which has been deposited simultaneously with the removal of that 
which possessed the original crystal form; it is easy, therefore, to 
see that the molecular arrangement of the particles must be entirely 
different from that of the original crystal. Now, by the aid of 
polarized light, such changes are at once rendered apparent, and we 
thus possess the means of obtaining most important information on 
the metamorphism of rocks and minerals, of which ordinary light 
would afford no indication whatever. 
Serpentine has hitherto been a great puzzle to geologists, some 
haying regarded it as an intrusive igneous rock, others as of meta- 
morphic origin. As not unfrequently happens, both are, I believe, 
right, for every section I have made clearly proves it to be an 
altered rock, and one specimen from the Vosges mountains contains 
numerous grains of olivine, in which the change is only partially 
developed. 
These few facts will serve to indicate the importance of this 
hitherto neglected method of inquiry; for although the pseudo- 
morphism of many minerals has been long studied, little attention 
has been directed to similar changes in rock masses. 
A subject of interest to the microscopical observer, and one of 
considerable importance to the petrologist, is the occurrence of 
minute fluid cavities in the minerals of igneous and metamorphic 
rocks; they have been detected in several minerals ejected from 
active volcanoes; but so far as I have observed, they are far more 
abundant in quartz than in any other mineral. Those who wish to 
examine them may do so by making a section of almost any speci- 
men of granite; they are very numerous in the granites and. schorl 
rocks of Cornwall, the hornblendic granite of Mount Sorrel, in the 
syenite and gneissoid rocks of Malvern, and in the syenite of Croft 
Hill, and neighbourmg bosses in Leicestershire. In these and 
similar rocks, the fluid cavities appear to be so entirely restricted 
to the quartz, that I have not yet detected any in the felspar or 
mica; they are certainly extremely rare in these minerals, if they 
occur at all; this, if established, would indicate a difference in the 
condition under which the minerals were formed, a point which 
I believe has not yet received attention. 
For an account of the curious spontaneous movements of the 
fluid in some of these cavities, and for other interesting matter con- 
nected with the subject, I must refer you to Mr. Sorby’s paper 
already quoted. 
During the past summer and autumn (1869) I have collected 
specimens of the igneous rocks of the Midland coal-fields from the 
following localities :—Kinlet and Shatterford, west of Kiddermin- 
ster; the Clee Hills; Little Wenlock, near the Wrekin, in Shrop- 
shire ; Coalville, near Bardon Hill, in Leicestershire ; and Matlock, 
in Derbyshire. 
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