Campbell. — On Crystalline Rocks. 

 Table of Changes produced by Metamorphism. 



451 



Earthy. 



Mud, Clay, Shale 



Sand 



Calcareous Mud 



Volcanic Ash and 

 Tuff .. 



Indiu'atecT, and in •sy-'hich 



cleavage is shown by 



slaty rooks. 



Claystone, Clayslate 

 or "Killas" 



Sandstone, Grit 

 Chalk, Limestone 



Slates . . 



Micro-crystalline, Schistose 

 (foliated). 



Argillaceous Mica- Schist,"^ 

 Talc Schist 



Quartzite, Quartz Schist, 

 Felsite (Petrosilex) 



Marble, Dolomite 

 Felstone Trachyte 



Crystalline. 



Foliated. Massive, 



vl Gneiss 



Granite 



The completeness of the change may vary from the incipient form shown 

 by indurated clay- slate rocks in cleavage, which always occur in a direction 

 other than the plane of bedding. Some slaty rocks are apparently volcanic 

 ash deposits. This structure of cleavage is more mechanical than chemical, 

 and caused by great lateral pressure by which the component particles were 

 flattened, producing lines of weakness at right-angles to direction of pres- 

 sure. This structure has been artificially produced in some soft substances, 

 by Dr. Sorby and Prof. Tyndal, and also by Messrs. Fox and Hunt, by 

 passing galvanic currents through masses of moistened pottery clay (Page's 

 Geology, p. 154). The contorted condition of fossils that occur in the 

 slates show also the disturbance of the particles forming the slate. 



Limestones pass from an indurated into a compact and microcrystalline 

 texture, becoming granular when highly metamorphosed. The latter gener- 

 ally occurs associated with schist, or in the proximity of eruptive rocks. 

 The whole of these varieties are popularly termed marbles. Crystalline 

 marble has been artificially produced by heating chalk under pressure 

 sufficient to prevent the escape of carbonic acid gas. Many accessory 

 minerals, such as zircon, spinel, corundum, lapis lazuli, are found in crys- 

 talline limestones. 



Arenaceous rocks, such as sandstones, are composed of rounded particles 

 of quartz ; and grits of angular fragments and crystals, together with 

 rounded particles of quartz ; these are converted by metamorphosis 

 into quartzites, the component particles being cemented with siliceous 

 material ; both kinds often contain felspar, which renders them capable of 

 conversion into felstone rocks, and when mica is present, micaceou.s kinds 

 are produced. 



The alteration passes then into foliation, which is a segregation into 

 crystalline layers of different mineral composition, the planes of separation 

 being either along those of the bedding or cleavage. This structure is 



