A CATALOGUE OF OBSERVATIONS OP LUMINOUS METEORS. 137 



of minerals are given in the figures at the end of this paper. These consist of, 

 fluid cavities, glass cavities, gas cavities, and stone cavities, according to the 

 different materials by which the cavities were filled in deposition. The 

 coexistence of all of these in the crystalline minerals of igneous rocks, is a 

 certain proof of the combined influence of igneous fusion, aqueous solutio n 

 and gaseous sublimation in the process of their consolidation. The fluid 

 commonly enclosed in mineral crystals is an aqueous solution of chlorides of 

 potassium and sodium, sulphates of potash, magnesia and lime, and free 

 hydrochloric or sulphuric acid. It forms in some specimens of quartz, fully 

 one per eent. of the total weight of the mineral. 



If for various specific reasons a dull red heat (680° F.) is adopted as the 

 ordinary temperature of consolidation of the igneous rocks, lavas, trachytes, 

 and granites, a fluid pressure of 4000 feet of superincumbent rock would 

 exactly equipoise the vapour-tension at this temperature of the saline fluids. 

 If a greater liquid pressure prevailed than this, which would generally be 

 the case, no vacuity could be formed within the cavities, until by cooling 

 and contraction the liquid pressure within them should decrease so far as not 

 to exceed the vapour-tension of the fluid. If p be the excess of pressure at 

 the time of consolidation at a temperature of 680° F., measured in feet of 

 superincumbent rock, v the observed proportion of vacuity to liquid volume 

 in the cavities at 0° C. (32° F.), the equation between these quantities, 

 derived from the best data of the elasticity of water, and of its vapour, in 

 combination with saline substances, is briefly 



A.Q 



^=369,0003^^ (10) 



If P be the total pressure of superincumbent rock, including the vapour- 

 tension at 680° F. at the time of consolidation, 



P= 4000 +28,385 (0-3-«). 

 The contraction of the fluids can be observed with considerable accuracy by 

 the aid of the microscope, in cavities that are equiaxial, or else (as in figs. 50, 

 101) in such as are long and slender, or very thin and flat. The value of v 

 varies in different specimens of quartz from 0-04 to 0-25, and from these the 

 following Table was calculated of depths at which a variety of igneous rocks 

 submitted to the microscope were probably consolidated : — 



Depth in feet. 



Trachyte of Ponza 4,000 



Elvan of Gwennap 18,100 



Granite at St. Austell 32,400 



Mean of Cornish Elvans 40,300 



More recent veins of Aberdeen Granite . . . 42,000 



Mean of Cornish Granites 50,000 



Elvan, at Swanpool, near Falmouth 53,900 



Granite from Ding-Dong Mine, Penzance . . . 63,600 

 Mean of the Highland Porphyry Dykes .... 69,000 

 Exterior of the main mass of the Aberdeen Granite 69,000 



Mean of the Highland Granites 76,000 



Centre of the main mass of the Aberdeen Granite 78,000 



Mean of all the igneous rocks . . . .49,692 

 The greater depths of the Aberdeenshire granites over those of Cornwall 

 (figs. 100, 116), shown in this Table, are conformable to general opinions 

 concerning these granites, derived from purely geological considerations. 

 1865. T 



