| OCR Text |
Show 102 GEOLOGY OF THE HIGH PLATEAUS. sometimes wanting. The accessory minerals are, with rare exceptions, far inferior to the essential ones in respect to quantity. The following conspectus exhibits these minerals: CONSPECTUS OF MINERALS CHARACTERISTIC OF THE PRIMARY DIVISIONS OF VOLCANIC ROCKS. Groups. Essential minerals. Accessory minerals. Group I. Orthoclaso (usually as sanidin) and free quartz. Hornblende, biotite, plagioclase. Group II. Orthoclase (usually as sanidin). Hornblende, biotite, augite,pla-gioclase (the latter seldom wanting), nepbelin (in pho-nolite), magnetite. Group III. Sub-acid rocksâ€"Andesites (including propylite). Hornblende, augite, biotite orthoclase (in subordinate quantity and seldom wholly absent), magnetite. Group IV. Plagioclase (in some cases replaced by leucite or nephelin), augite. Olivin, magnetite. In addition to the minerals presented in the foregoing scheme, there remain several others of considerable importance. These are chiefly leucite and nephelin. Leucite is found in some basalts replacing the feldspar, and is treated in the classification precisely as if it were plagioclase. Though widely distinct from that group of minerals in its crystallographic forms, it closely approaches them in chemical constitution, differing in this respect mainly in containing a little higher percentage of potash than normal orthoclase. Nephelin holds exactly the same relations and presents the same distinctions, but holds a high percentage of soda instead of potash. It is found not only in the basalts, but also in phonolite, and is generally held to be the most characteristic mineral of the latter rock. If now we treat these two minerals as just so much triclinic feldspar, we shall find no diffi- |
