138 



REPORT 1865. 



The mean depth of consolidation of all the igneous or gneissoid rocks of the 

 Table is nearly 50,000 feet, and a rate of increase of 1° in 85 feet, according 

 to the information of Mr. R. Hunt, observed in descending to the bottom of the 

 deepest mines in England, would require a temperature of 680° F. (adopted in 

 calculating this Table) at a depth below the surface of 53,500 feet, very nearly 

 in accordance with the original supposition. 



The molten lava of Mount Teneriffe can hardly be supposed to have an 

 actual depth less than that of 30,000 feet. The height of the mountain 

 and the depth of the ocean near its western side, indeed, amount together to 

 28,000 feet, and if the lava have any lateral extension to a moderate distance 

 in the same direction, a few thousand feet of rock must necessarily intervene 

 between its surface and the ocean-bed. The igneous rocks, hitherto examined 

 in the microscope, appear therefore to have been consolidated at depths com- 

 mensurate with those of modern lavas. In other words, granites, trachytes, 

 and gneissoid rocks are unerupted lavas of older volcanoes, consolidated by 

 loss of temperature, and variously protruded to the surface along with the 

 superincumbent strata. 



50 



c 



Explanations of the Figures. The sign of multiplication indicates the ampli- 

 fying power employed ; in linear measure. 



Fig. 2. (p. 136). A portion of the growing edge of a crystal of chloride of 

 sodium, x 200 : showing how the fluid cavities are formed. The shaded part 

 represents the concentrated solution entering into a deep notch formed by the 

 irregular growth of the crystal. The notch may be enclosed by a plane sur- 

 face, or, as shown by the dotted line, by the further growth of the crystal, 

 and cavities like those upon the right or left of the figure will be the result. 



Fig. 50. A fluid-cavity. 



Fig. 80. A fluid-cavity in the nepheline of a block ejected from Vesuvius. 

 X 1000. 



Figs. 81, 82. The cavity, fig. 80, after having been subjected to a more 

 or less bright red-heat. 



Fig. 86. A gas-cavity in the nepheline of a block ejected from Vesuvius, 

 in a natural state, x 1000. 



Fig. 90. A glass-cavity in the nepheline of a block ejected from Vesuvius, 

 X 1200. 



Fig. 91. The cavity, fig. 90, after having been subjected to a bright red heat. 



Figs. 100, 101. Fluid-cavities in the quarti of a Trachyte from Ponza; 

 100. x 2000 ; 101, x 800. 



