342 J. ARTHUR PHILLIPS ON THE ROCKS OF 



that the crystal of that substance must have become cracked while 

 still in a somewhat plastic condition, and that the schorl was thus 

 enabled to penetrate into its substance. In support of this hypo- 

 thesis may be adduced the facts stated in connexion with the for- 

 mation of stone-cavities in the quartz crystals of elvans (p. 337), 

 the apparent crystallization of opaline silica at Borax Lake, Cali- 

 fornia (p. 341), and the occurrence of curved crystals of quartz, 

 described by Breithaupt ('Die Paragenesis der Mineralien/ p. 11) 

 as having been procured from a druse in the Grisons. 



Some of the fluid-cavities of veinstones, like those of rocks, con- 

 tain no visible bubbles ; but they frequently enclose acicular crystals 

 of schorl, and cubes which are probably common salt. 



General Conclusions. 



The various facts which have been described will, I think, warrant 

 the following general conclusions : — 



1. The clay-slates of Cornwall vary materially in their composi- 

 tion ; but under no circumstance could the mere re-arrangement of 

 the constituents of any one of those examined, or of any number 

 of them, result in the production of granite. 



2. Some of the " greenstones" of the Ordnance Geological Map 

 are volcanic rocks contemporaneous with the slates among which 

 they are found, and possessing the chief distinguishing features of 

 modern lavas. Others are hornblendic slates, &c. 



3. Granites and elvans, having a similar chemical and mineralo- 

 gical composition, were probably derived from the same source ; but 

 the volume of the bubbles in the fluid-cavities of both, having no 

 constant relation to the liquid, docs not afford any reliable data from 

 which to calculate the temperatures at which these rocks were 

 respectively formed. 



4. The stone-cavities of elvans, and probably of some other rocks, 

 are sometimes the result of the irregular contraction (previously to 

 the solidification of the base) of imbedded crystals of quartz. In 

 rocks having a glassy base the final result will be glass -cavities. 



5. The vein-fissures of the tin- and copper-bearing lodes of Corn- 

 wall were generally the result of forces acting subsequently to the 

 solidification of the elvans, but operating in the same general direc- 

 tion as those which caused those rocks to be erupted. 



6. The fissures thus produced afterwards became filled with mine- 

 rals, resulting from deposits, by chemical action, from waters and 

 aqueous vapours circulating through them. From coming into 

 contact with highly heated rocks at great depths, these waters were 

 sometimes at a high temperature ; but it is probable that, in many 

 cases, the heat was very moderate, and the action comparatively 

 slow. 



7. To what extent these deposits were produced by waters rising 

 from below, and how far they were influenced by lateral percolation 

 cannot be determined. The effects produced on the contents of veins 

 by the nature of the enclosing rock, and the frequent occurrence of 



