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compact felspar: 381 Gcognoslic Situation. This mineral occurs in mountain-masses, beds and veins, either pure, or intermixed with other minerals, m Primitive, transition, and floetz rocks. In primitive mountains, it is associated with hornblende in greenstone, and greenstonc-slate; and it forms the basis of several felspar-porphyries. ISeds of it in a.pure state occur in gneiss, and other primitive rocks. In transition moun tains, it occurs in beds, as a constituent part of porp y ry and greenstone ; and beds of it occur either pure, or in Porphyry, or greenstone,'in floetz mountains. Geographic Situation. The Pentland Ilills contain beds of compact felspar, associated with claystone, old red sandstone, and conglo merate. It occurs in a similar situation on the hill of Tinto, described by Dr Mack night in the 2d volume of the Memoirs of the Wernerian Society. Mr Mackenzie found it along with floetz rocks in the Ochil Hills * ; and Mr Fleming observed it associated with rocks of the same nature in the Island of Papa Stour, one of the Zetland group +. Reds of it, which are sometimes porphyritic, occur in the transition rocks of Dumfriesshire and Gal loway ; and in rocks of the same class to the north of the Frith of Forth, as in Perthshire, and the Mearns %. In the primitive rocks to the north of the Frith of Forth, it occurs in beds and veins, either pure, or in the state of porphyry. Examples of both occur in Perthshire, in the course * Memoirs of the Wernerian Society, vol. ii. p. 20. i* Memoirs of the Wernerian Society, vol. i. p. 170. $ Imrie, in Transactions of Royal Society of Edinburgh, vol. VI,