di 
Moe tae Bs ie 
layers 
ae — 
CampsELi.—On Crystalline Rocks. 451 
Taste or CHANGES PRODUCED By MrTAMORPHISM. 
‘ ; Crystalline. 
Indurated, and in which x s 
Earthy. ’ is sh Micro-erystalline, Schistose |——— 
. ee ee (foliated). Foliated.| Massive 
Mud, Clay, Shale} Claystone, Clayslate 
or “ Killas” .| Argillaceous Mica- esnsa: 
Tale Schist 
Quartzite, Quartz Se 
Band <i. .-| Sandstone, Grit .. { Felsite Girodiesy 
Calcareous Mud | Ghalk, Limestone ..| Marble, Dolomite 
Gneiss | Granite 
Voleanic Ash and 
pi ees .-| Slates.. se ..| Felstone Trachyte 
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 erys- 
talline limestones. 
Arenaceous rocks, such as sandstones, are composed of rounded particles 
of quartz; and grits of angular fragments and erystals, 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, micaceous kinds 
are produced. 
The alteration passes then into Se paeroention: Sve 
crystalline of different mineral omposition, the planes separation _ 
OF mene De eee is 
