ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 177 



deposits with wMch we are particularly concerned were penetrated 

 by eruptive rocks while still far below the surface, and conse- 

 quently subject to heat and pressure corresponding to many 

 thousands of feet of overlying rock. It is not surprising, there- 

 fore, that effects have been produced entirely different. For 

 example, when chalk is heated by a lava-stream in such a 

 way that the carbonic acid can escape, it is merely converted 

 into quicklime. But if it be similarly heated in the depths of 

 the earth under great pressure, it is converted into granular 

 limestone or crystalline marble. | 



In the changes just referred to moisture does not seem to 

 have played any important part. But, as we have already seen 

 in the preceding section, the solvent powers of the fluids circulat- 

 ing through the rocks are greatly increased and extended 

 by increase of heat and pressure, so that substances ordinarily 

 regarded as insoluble have been brought into solution, transferred 

 to other points, and finally re-deposited in suitable gites. Thus 

 silicates have been decomposed and dissolved — new silicates have 

 been formed — free silica has been deposited — and the separated 

 bases have been converted into sulphides, sulphates, and other 

 comparatively stable compounds ; while the haloid salts of 

 alkalies, derived in all probability from pre-existing complex 

 silicates, have gradually accumulated in the waters at the earth's 

 surface. 



In the West of England, it is doubtful whether effects of 

 heat and pressure can be anywhere seen without the superadded 

 effects of other changing agencies yet to be described. It seems 

 certain, for instance, that such flinty slates and sandstones as 

 those of Haytor, described in Sec. 2, have not only been consol- 

 idated by pressure and baked by heat, but also infiltrated with 

 silica ; while the crystallizing forces have developed within them 

 new minerals, such as magnetite, garnet, and hornblende ; the 

 materials of which were in all probability already present in the 

 rock. 



t The result of such heating may be seen in many places in the North of 

 Ireland, where the chalk, being penetrated by dykes of basalt, is altered into a 

 hard grey semi-crystalline limestone, or into a coarsely crystalline white marble. 

 The well-known experiment of heating chalk in a closed gunbarrel has a similar 

 result. 



