The following are the rocks of this class : 1. Grey-wacke.
2. Transition limestone.
3. Granite and porphyry.
4. Gneiss, mica-slate, 8a.
5. Serpentine.
6. Quartz-rock.
7. Red sandstone.
8. Transition-trap.
9. Gypsum.
I. Grey-wackr.
Grauwacke.—TVerner. Greywacke.—Jameson. Traumate.—Daubuisson. Psamite.—Brongniart.
Breche, Poudingue, and Gres, of some French geologists.
Composition.—It is composed of angular or other portions of quartz, felspar, Lydian-slate, and clay-slate, connected together by tneans of a basis or ground of the nature of clay-slate, which is often highly impreg nated with silica, thus giving to the mass a considerable degree of hardness. The imbedded portions vary in size, but seldom exceed a few inches in breadth and thickness. When the imbedded portions become very small, the rock assumes a slaty structure, and forms the Grcy-wacke slate of geognosts. When the grains al most entirely disappear, and the rock is principally composed of clay-slate, it is named transition clay-slate. This clay-slate has frequently a much more earthy as pect than the varieties found in the primitive districts. Besides the grey-wacke already described, which has the conglomerated structure, another, having the same structure, is occasionally met with in transition districts.
It has been named transition conglomerate, or pudding stone. It is composed of roundish or angular masses of granite, porphyry, gneiss, and clay-slate, often larger than a man's head, imbedded in clay-slate, or nearly without a basis or ground.
Common grey-wacke does not occur so frequently as greywacke slate and transition clay-slate.
Subordinate beds.—Transition clay-slate, and grey wacke, contain occasionally different kinds of rocks in subordinate beds, or in veins. The following are the principal of these : 1. Quartz.—It occurs in beds, imbedded masses, and veins, and frequently in very considerable quantity.
2. Talc.—This mineral occurs in imbedded masses, and in layers in transition clay-slate. There is a fine example of it in the clay-slate of Glaris, in Switzerland, which is remarkable for the petrified fishes it contains.
3. TVhet-slate.—This mineral forms beds in the clay slate.
4. Serpentine—Beds of serpentine, often of great thickness, and of considerable extent, occur along with transition clay-slate.
5. Lydian stone.—Beds of this rock occur in clay 6. Alum slate.—This rock is clay-slate impregnated with carbon and sulphur. The sulphur is either com bined with the carbon, or united with iron, forming iron