METAMORPHIC AND IGNEOUS ROCKS. 611 



felspai", hornblende, chlorite, and other minerals, which must have 

 been produced by re-arrangement of the substances contained in tlie 

 sediments, under the influence of chemical agencies. Such fossils 

 as may have existed in these rocks have entirely disappeared, or may 

 in some instances be seen to be replaced by mere nests of Calcareous 

 crystals or even by crystals of garnet. In the district now under 

 consideration, these metamorphic rocks may be grouped under the 

 following heads, each of which, however, includes many varieties 

 graduating into each other: — (1.) Quartiite, or a hard silicious rock 

 produced from the alteration of sandstone or arenaceous shale; (2.) 

 Gneiss, a stratified rock composed of quartz, felspar, and hornblende, 

 or quartz, felspar, and mica, and a product of the metamorphism of 

 conglomerates and other mixed sediments; (3.) Micaceous and 

 chloritic slates, consisting of quartz and mica, or quartz and chlorite, 

 and apparently a result of the further alteration of clay slates, into 

 which the micaceous schists graduate; (4.) Diorite or Hornblendic 

 greenstone, a crystalline mixture of the mineral hornblende with 

 felspar, often laminated or rudely stratified. These rocks may be 

 merely altered mud-rocks or shales, but in many cases they may have 

 been originally volcanic tufas or ash-rocks. (6.) Compact Felspar 

 and Felspar porphyry, containing small isolated felspar crystals in a 

 paste of more compact material of similar composition. Many rocks 

 of this character appear to be stratified, and are probably metamorphosed 

 clays. (7.) Crystalline Limestone or Marble, usually white and some- 

 times with crystals of tremolite and patches of serpentine. Owing 

 to the small amount of calcareous matter in the original sediments, this 

 kind of rock is not largely developed in the Upper Silurian districts. 

 All of the above are stratified and metamorphic. With these are 

 other rocks in masses or veins either intrusive, and of the nature of vol- 

 canic rocks, or "indigenous" products of the fusion of sedimentary 

 rocks in situ. These igneous rocks, as they may be called, though 

 probably of similar origin with the trap of the Triassic and Carbon- 

 iferous systems, differ somewhat in composition and appearance. They 

 are mostly coarser grained or more crystalline, indicating that they 

 are less superficial, and hence have cooled more slowly. Hornblende 

 usually replaces the Augitc of the trap. Felspar, which is the pure 

 white or flesh-coloured part of ordinary granite, exists in greater 

 abundance than in trap. Quarts or uncombined silica is also often 

 present in considerable quantity. Uocks of this class are very variable 

 in their composition and appearance, hence it is difiicult to give them 

 accurately distinctive names, and geologists entertain diff'erent opinions 

 as to the amount of internal heat, or igneous action proper, involved 



