416 



p. 315) into greisen containing topaz ; the occurrence of pseudomorphs 

 of tourmaline and quartz, and of cassiterite and quartz after felspar ; and 

 the association of tourmaline with the masses of decomposed granite 

 from which the kaolin is derived. 



In 1824, LEOPOLD VON BUCH maintained that the deposits of 

 kaolin in the neighbourhood of Halle were formed by the action of 

 hydrofluoric acid on porphyry, and instanced the occurrence of 

 fluorite in the decomposed mass as evidence of his view. M. DAUBREE W 

 and Mr. COLLINS have applied the same theory to account for such 

 masses of kaolinized granite as those occurring near St. Austell. 



Mr. COLLINS has shown by experiment that kaolin may be produced 

 by the action of hydrofluoric acid upon felspar. The proof, therefore, 

 that kaolinization may be produced by subterranean as well as by surface 

 agencies appears to be of a very satisfactory nature. 



As a further illustration of the metamorphosis of igneous rocks by 

 exhalations of compounds of fluorine and boron we may refer to the 

 conversion of ordinary granite into shorl-rock, greisen, luxullianite and 

 trowlesworthite. In all these cases the metamorphism is in all proba- 

 bility effected immediately after the formation of the rock by the 

 fumarole-action which is a part of the phenomenon of intrusion. The 

 case is similar to the alteration of a lava by the gases and vapours 

 oiven off during the final stages of consolidation. It is, indeed, im- 

 possible, to separate by any hard and fast line the phenomena 

 of consolidation and those of metamorphism. One part of a mass 

 may be undergoing change while the adjacent parts are still 

 unconsolidated. 



As regards the alteration of igneous rocks by the intrusion of 

 other rocks there is little to be said. The only case which has been 

 worked out in anything like detail is the effect of granite on basic 

 igneous rocks, and this has already been described at length. 



We come now to the last type of deep-seated metamorphism, viz., 

 that due to the earth-stresses. 



Having regard to the great importance of the subject of dynamic 

 metamorphism from a petrographical point of view, and to the fact 

 that we are only beginning to recognize the structural and mineralogical 

 characters which are to be attributed to it, a few general remarks on 

 the nature and distribution of this kind of metamorphism will not be 

 out of place. It must, however, be distinctly understood that these 

 remarks are of a more or less speculative character. Our ideas on the 

 subject are " en pleine evolution," and liable, therefore, to rapid 

 modification and improvement as our knowledge increases. 



The movements of elevation, depression, and lateral compression 

 produced by the stresses which arise in the earth's crust have compelled 

 rock-masses to assume forms other than those which they possessed in 

 the first instance; that is immediately after their deposition and con- 



(1) Geologie Experimentale, p. 64, 



