DI'ABASE (from (k. doable a 2oing. from to go: inlIneneed in lona by I:I:. diobaxis, pas from dig. through 4- Ifirtic, basis. it going. ha-el. A crystalline, igneous rock of which the essential constituent minerals are feldspar and pyroxene (augite). texture of diabase. while generally crystalline throughout. differs from that of granite in that the aligne inclose. the feldspar. owing to the later period of augite crystallization. The feld spar of diabase is usually in bladed crystals, which take all directions in the augite and give the rock it peculiar interlocking radial texture. which is especially noticeable in roc(., of coarse grain. the grains of augite he large in com parison with the size of the feldspar blades, the effect produced is a noteworthy mot which is described as 'lustre mottling.' The average chemical composition of diabase varies lout little from that of gabbro (q.v.), the difference of the two rocks being chiefly of structural nature. The closest affinities exist both chemically, mineral ogically„ and texturally between diabases and basalts (q.v.).:Ind. indeed, moat .Ain•rican pet rog•
raphers recognize no important difference be tween them; the rock- of greater geologic age, which consequently show greater decgolip.isition, being, however. more generally described under the term diabase and the more recent oreur• TenVes as basalt. Olivine, if present in a diabase, makes the rock au olivine diabase. 1)inbase is quite subject to alteration. the augite being re placed by hornblende and the olivine (if pres• (tit 1 by serpentine. Both of the newly devel oped minerals being green, the rock assumes by this alteration a greener color. important occur rences of diabase are: those of the copper-bear ing Neweenawall "writ: of Lake Superior; the Palisades of the lower Hudson River, and the Hanging llills of Meriden, C'onn.. of •Iura-Trias sie Age; and those of the Deccan plateau of southern India. :see thil.:ENSTOXE: BASAI:T; Ta.te.