APPENDIX—HALL'S EXPERIMENTS. 145 



known to both the chemist and the geologist, having to the 

 latter afforded explanations of appearances of no little com- 

 plexity, and to the interest excited by his investigations are, 

 in no small degree, to be traced many discoveries which have 

 been made by Continental philosophers. To the geologist and 

 chemist of the present day, the experiments of Hall tell that a 

 vast field is open for investigation ; and examinations, philoso- 

 phically conducted on the grand principles which he chalked 

 out, would, it may with certainty be stated, either corroborate 

 the truth of certain, as yet, theoretical points in geology, 

 or display their falsity. By furnace experiments knowledge 

 could be gained, in all probability, on much which is now 

 the " Terra Incognita " of geology. How is dolmitization 

 effected ? Are the granites, the porphyries, the trap-rocks, 

 trachytes, and lavas to be considered only as links in a con- 

 nected chain of igneous elaborations, of which the one ex- 

 tremity is granite and the other lava, the differences in 

 mineral structure and composition being effected by diffe- 

 rences in compression, and in the rocks with which they 

 have come in contact ? Do the various stratified deposits 

 contribute to the formation of igneous rocks ; and can 

 sandstones, shales, and conglomerates be made to assume 

 characters indicative that the formations of gneiss, mica- 

 slate, hornblende rock, and the several strata denominated 

 Primitive, are only to be considered as mechanical deposits, 

 which have been completely altered ? All these are ques- 

 tions which it is of the highest importance to solve, and so- 

 lutions appear only to be got in experimenting on mineral 

 masses, at very high temperatures, and under great com- 

 pression. 



