HARD WICKE'S S CIENCE - G SSI P. 



265 



QUARTZ, AS IT OCCURS IN THE LAKE DISTRICT: 

 ITS STRUCTURE AND ITS HISTORY, 



PART III. 



By J. CLIFTON WARD, F.G.S., F.R.M.S., &c. 



FEW years ago, being 

 anxious to apply Mr. 

 Sorby's method to the 

 granites and granitoid 

 rocks of the lake- 

 country, I examined a 

 number of slices of the 

 granite rocks, and mea- 

 sured nearly fivehundred 

 cases of liquid-cavities, 

 ascertaining the relative 

 size of the vacuity (or bubble) to the liquid-cavity. My 

 method of proceeding I will describe directly, but I 

 will at once state that the result arrived at in the 

 case of the Skiddaw granite, for example, was, that 

 its consolidation took place under a pressure of rather 

 more than an equivalent of 51,000 ft. of rock, and 

 that the mean pressure under which all the principal 

 granitic and granitoid rocks were consolidated was 

 equivalent to that of 44,000 ft. of rock. 



My experiences in the course of this investigation 

 may be of interest to some who wish to undertake 

 original work of a similar kind. (For the Memoir 

 containing the investigation as a whole see Quar- 

 terly Journal of the Geological Society, vol. xxxi., 

 p. 568.) 



The object-glass generally used was a i-in. (of 

 Collins), with a C eye-piece, the combination magni- 

 fying 665 times. When I first began my measure- 

 ments I used a neutral-tint glass reflector, and traced 

 on paper all the best-defined fluid-cavities, making a 

 note by the side of those in which the vacuities 

 showed a constant spontaneous movement. I soon 

 found, however, that on a comparison of a consider- 

 able number of drawings, the fixed bubbles were 

 almost invariably relatively larger than those which 

 showed this free movement. Hence I was led al- 

 together to reject from my measurements all cases in 

 which the bubble was fixed, and in which it would 

 seem either that gas had been primarily entrapped, 

 No. 1 68. 



or that in the making of the thin slice leakage had 

 occurred in the cavity. This last case must neces- 

 sarily often occur in the making of thin slices, espe- 

 cially as the liquid-bearing cavities are frequently so 

 irregular in shape and prolonged into horns and 

 fine points. I then gave up tracing the outlines of 

 cavities on paper, and measured the relative size 

 of bubble and liquid-cavity directly by means of a 

 Jackson's micrometer, with divisions equal to T o o 00 th 

 of an inch. This micrometer being placed in the 

 eye-piece, the fine divisions could be brought over 

 the bubble and liquid-cavity, and their relative size 

 at once estimated with tolerable accuracy. But a 

 little consideration will make it evident that measure- 

 ment of a liquid-cavity in one plane would be of 

 little use unless the cavity be exceedingly shallow 

 and lie along that plane. Hence it became neces- 

 sary to take this further precaution, viz., to rely only 

 upon the measurements of those cases in which the 

 tiny bubble (or vacuity) moved freely into all parts ot 

 the liquid-cavity without going out of focus ; this 

 would imply that the cavity was of tolerably uniform 

 depth, but little more than the depth of the diameter 

 of the bubble. And it was found that, when I 

 restricted my measurements to these cases, there was 

 a fairly-marked uniformity in the ratio between 

 vacuities and liquid-cavities occurring in the quartz of 

 the same rock. 



Thus, take as an example two different rock-slices 



of the same granite 



•154 

 •154 

 •154 

 •160 

 •180 

 •180 

 •154 

 '154 



No. 1. 



•162. 



No. 2. 



V 



/ 



/•166 

 •125 

 •166 

 •180 

 •200 

 •166 

 •166 

 •166 



■154 



•166 

 •142 



V142 



•164. 



