434 Reports and Proceedings. 



nearly always easy to obtain some, wMch, being emt at an inclination 

 to the optic axis, exhibit different degrees of intensity of action ; 

 therefore minerals which vary much from each other in this respect 

 may be easily distinguished. 



A most important point to be noted is, that the depolarized action 

 of a crystal is uniform over the whole surface of its section if it 

 consists of one simple crystalline structure ; when, however, the 

 light appears to break up into detached parts, each of which changes 

 independently as the analyzer is rotated, we know that it is made up 

 of a number of separate crystalline portions, either independent of 

 each other, or sometimes related as twins.^ 



A knowledge of these facts enabled me to detect the presence of 

 olivine and its pseudomorphs in the Eowley rock, as described in a 

 former paper, and also in the GtEological Magazine, Vol. VI., 

 p. 115. A pseudomorph is a mineral possessing a crystalline form, 

 which does not belong to the substance of which it is composed ; it 

 is an altered mineral, or in other words, an aggregate of mineral 

 matter, which has been deposited simultaneously with the removal 

 of that which possessed the original crystalline form ; it is easy, there- 

 fore, 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 

 having 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 nu- 

 merous 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 volcanos ; 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 specimen 

 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 neighbouring bosses in Leicestershire. In these and 

 similar rocks, the fluid cavities appear to be so entirely restricted to 



1 Sorby " On Examination of Eocks and Minerals" in Dr. Beal's Work on the 

 Microscope. 



