SURROUNDING THE LA^'D's-END MASS OF GRANITE. 413 



of interest, which it may be convenient to consider apart from the 

 descriptions of the rocks themselves. 



Quartz. — The strata near the granite are far more highly silicated 

 than those at a distance from it ; and there can be no doubt that 

 much of the quartz has been derived directly from the intruded 

 rock. 



In thin sections, showing the junction of the two rocks, there are 

 many microscopic quartz- veins extending from the granite across the 

 slate ; they are not fissures traversing both rocks, but commence at 

 the sharp line of junction, and are seen in the slate only : they are 

 continuous with the quartz of the granite, and sometimes contain 

 small quantities of felspar or mica. 



A great part of the quartz may nevertheless have been produced 

 by a more or less complete decomposition of the constituents of the 

 original sedimentary deposit ; the aluminous and other portions 

 would then be removed, leaving the silica free to rearrange itself 

 and form the separate folia. That movements of this kind have 

 taken place, and that the mass was at one time in a plastic state, is 

 evident from the fact that the quartz was often injected into fine 

 fissures, and very frequently has enclosed in it crystals of tourmaline 

 and mica. 



One of the most interesting points connected with this mineral is 

 the number of minute fluid-cavities with which it is so frequently 

 crowded. 



Fluid-cavities. — Fluid- cavities are abundant in the quartz of the 

 large masses of granite, in that forming the veins, in the schorl rock, 

 and in the altered slates. 



I have examined very many of these cavities with excellent objec- 

 tives, and have satisfied myself beyond a doubt that there is very 

 great irregularity in the relative sizes of the cavities and vacuities, 

 not only in different parts of the same mass of rock, but even in 

 single crystals or grains of quartz. 



A remarkable example occurs in the granite in contact with 

 altered slate at Mousehole ; in a single grain of quartz there are 

 several fluid-cavities containing active bubbles, three of which ex- 

 hibit widely different proportions to the size of the cavities. 



There are also not a few cavities which appear to be quite filled 

 with a fluid of similar refractive power to that containing bubbles. 

 There are also very many cavities containing cubic crystals in addi- 

 tion to liquid and vacuity ; these are shown by Mr. Sorby to be 

 either sodic or potassic chloride, and are no doubt preserved in a 

 saturated solution ; the fluid is probably therefore of a different den- 

 sity from that in which there are no such crystals. That- the den- 

 sity of the liquid varies considerably in different cavities is quite evi- 

 dent, however, from direct observation ; for on rotating the stage of 

 the microscope in order to cause the bubble to move from one end 

 of a cavity to another, the motion is comparatively rapid in some, 

 while it is so sluggish in others as to be more like the rise of a 

 bubble in oil than in water. 



Considering these difficulties, and the impossibility of obtaining 



