METAMORPHLSM OF ROCKS 1077 



shown that in portions of the Roche Castle rock, in Pembrokeshire, 

 the porphyritic felspars have been replaced by quartz. The 

 tourmaline, gilbertite, and other minerals often found at or near 

 the junction of granite and sedimentaries (e.g. in parts of Cornwall 

 and Devon) are probably results of hydrothermal metamorphism, and 

 in this way many metallic ores may be deposited ; while the conver- 

 sion of peridotites into serpentines, sandstones into quartzites (not 

 to mention other instances), are results of the action of water, 

 probably with some slight increase of pressure and temperature. 



The intrusion of an igneous Jrock generally has an important 

 influence on the structure and mineralogical composition of the 

 surrounding mass, portions of which it can include and partially 

 dissolve (contact-metamorphism). Sections from the junction of an 

 igneous rock with one of sedimentary origin are highly interesting. 

 The metamorphism is found to consist largely in the development of 

 new minerals, such as chiastolite, andalusite, brown and white mica, 

 garnets, staurolite, etc. ; the first and third of these appear to form most 

 readily, andalusite after a time replacing chiastolite ; while the last 

 three require high temperatures. Gradually the original sedi- 

 mentary structure disappears from a rock affected by contact- 

 metamorphism, and one truly crystalline is set up, which, however, 

 has characters of its own. 1 Limestone becomes crystalline, fossils 

 disappearing, and minerals such as wollastonite, idocrase, &c., are 

 formed from impurities. Occasionally the heat is so intense as to 

 fuse at least the matrix of sandstones into a brownish glass. 



The microscope has also proved most useful in studying questions 

 relating to dynamic metamorphism, or that due to ' earth-stresses.' 

 The deformation by movement has sometimes been so great as to 

 obliterate, partially or even wholly, the original structure of a rock. 2 

 The intense pressures must produce some elevation of tempera- 

 ture and increase the solvent action of water, so that the original 

 constituents of the rock are destroyed, partially, if not wholly, and 

 at a later stage new minerals are produced. It has been shown that 

 many gneisses and schists (though not all) have been formed by 

 crushing or shearing from igneous rock, e.g. gneiss from granite, 

 hornblende schist from dolerite. In the former case, the crushing 

 of the felspar, the formation of white mica and free quartz from 

 its dust, 3 the effects produced on the other minerals, can all be 

 studied under the microscope ; and in the latter the conversion 

 of augite into hornblende. This, however, maybe brought about by 

 more than one cause, and each probably produces effects which can be 

 distinguished. These questions, however, on which many experienced 

 petrologists have been engaged for at least fifteen years, are much 

 too difficult and technical to be discussed in a book of this character ; 

 enough to say that heat, pressure, and water, singly and conjointly, 

 produce important changes in rocks, many of which can now be 

 identified. 



1 Bonney, Quart. Journ. Geol. Soc. xliv. (1888), p. 11. 



2 Tresca, ' Flow of Solids,' Proc. Inst. Mech. Eng. 1878, p. 301. 



3 A minute hydrous mica, often called sericite, seems to form readily in an 

 argillaceous rock under pressure. The silky-looking slates (to which the name 

 phyllite is restricted by some authors) are largely composed of it. 



