558 HISTORY OF GEOLOGY. 



mineral masses lias been produced. These forces are probably identi- 

 cal, on the one hand, with the cohesive forces from which rocks derive 

 their solidity and their physical properties ' while, on the other hand, 

 they are closely connected with the forces of chemical attraction, No 

 attempts, of any lucid and hopeful kind, have yet been made to bring 

 such forces under definite mechanical conceptions : and perhaps mine- 

 ralogy, to which science, as the point of junction of chemistry and 

 crystallography, such attempts would belong, is hardly yet ripe for such 

 speculations. But when we look at the universal prevalence of crystal- 

 line forms and cleavages, at the extent of the phenomena of slaty 

 cleavage, and at the segregation of special minerals into veins and 

 nodules, which has taken place in some unknown manner, we cannot 

 doubt that the forces of which we now speak have acted very widely 

 and energetically. Any elucidation of their nature would be an im- 

 portant step in Geological Dynamics. 



[2nd Ed.] [A point of Geological Dynamics of great importance 

 is, the change which rocks undergo in structure after they are 

 deposited, either by the action of subterraneous heat, or by tV- influ- 

 ence of crystalline or other corpuscular forces. By such agencies, 

 sedimentary rocks may be converted into crystalline, the traces of 

 organic fossils may be obliterated, a slaty cleavage may be produced, 

 and other like effects. The possibility of such changes was urged by 

 Dr. Hutton in his Theory ; and Sir James Hall's very instructive and 

 striking experiments were made for the purpose of illustrating this 

 theory. In these experiments, powdered chalk was, by the application 

 of heat under pressure, converted into crystalline calcspar. Afterwards 

 Dr. McCulloch's labors had an important influence in satisfying 

 geologists of the reality of corresponding changes in nature. Dr. 

 McCulloch, by his very lively -and copious descriptions of volcanic 

 regions, by his representations of them, by his classification of igneous 

 rocks, and his comprehensive views of the phenomena which they 

 exhibit, probabJy was the means of converting many geologists from 

 the Wernerian opinions. 



Rocks which have undergone changes since they were deposited are 

 termed by Mr. Lyell metamorphic. The great extent of metamorphic 

 rock changed by heat is now uncontested. The internal changes which 

 are produced by the crystalline forces of mountain masses have been 

 the subjects of important and comprehensive speculations by Professor 

 Sedgwick.] 



