50 INTERNAL STRUCTURE. as is often the cafe with granite, fienite, and greea- ftone, this kind of ftrudture is formed. 4. Slaty-porphyritic.—This kind of ftrudture is flaty in the final], and porphyritic in the great. The bajis is ilaty, and the porphyritic ftructure is formed by interfperfed cryftals or grains of foflils. different from the bafis. Mica-flate, when it con tains grains or cryftals of garnet, is faid to have a flaty-porphyritic ftrudture. 5. Porphyritic and Amygdaloidal.—Here two kinds of ftrudlure are placed together. It occurs in many amygdaloidal and porphyritic ftones. When Amygdaloid contains, befides the elipti- cal-fhaped maffes, alfo cryftals of hornblende and mica, it is then faid to have an amygdaloidal and porphyritic ftru&ure ; the amygdaloidal being the predominant. In Bafalt, on the contrary, where the two kinds of ftru&ure fometimes occur, the porphyritic is the predominating. Green Porphy ry, although rarely, fometimes poflefies fuch a double ftru&ure. TABLE
