NO. 1715. OLIVINE-DIABASE FROM NORTH CAROLINA.— POGUE. 477 



The feldspar is basic labradorite, possibly running into bytownite, 

 and makes up about 45 per cent of the rock. It occurs in long, 

 slender laths of subhedral habit. The albite twinning is universal, 

 in combination with which is occasionally found a Carlsbad twin. 

 Zonary structure is not pronounced. Inclusions consist of small 

 particles of iron ore and rare shreds of biotite. The alteration of 

 the feldspars is insignificant. 



Augite, the most common ferromagnesian constituent, forming 

 about 35 per cent of the rock, is pale green in color and nonpleochroic. 

 It rarely shows crystal outline, but surrounds and incloses the 

 feldspars, forming a matrix in which they are arranged at random. 

 Iron ore occurs as inclusions, and the alteration product, of which 

 there is little, is a pale green, scaly mineral, probably antigorite. 



Olivine is present to the extent of about 17 per cent, and forms 

 rounded crystals or grains of a very pale color. It includes particles 

 of iron ore, and is partly changed to talc instead of serpentine, the 

 more common alteration product of olivine. This mode of decompo- 

 sition differs from the more usual form in that the change is not 

 confined to the cracks of the mineral, but proceeds independently 

 of these, often beginning within the crystal, and is not necessarily 

 accompanied by the segregation of iron ores. The formation of 

 talc from olivine is very unusual in an olivine-diabase, though among 

 the Appalachian peridotites this mode of alteration, termed "steatiti- 

 zation," is stated to be comparable to serpentinization.'^ 



The iron ores are abundantly scattered through the rock, but 

 prefer the company of the ferromagnesian minerals, particularly 

 olivine. They occur in both grains and specks, and rarely show 

 good crystal outline. 



Texture. — The diabase forms an excellent example of the ophitic 

 texture; that is, the feldspars are long, slender laths, arranged at 

 random, and around these the augite is molded. The olivine mostly 

 holds its own form against that of the plagioclase, but in some cases 

 it includes, or partly includes, a feldspar lath. Thus it appears 

 that the olivine in part crystallized previous to the feldspars, and 

 in part the crystallization was simultaneous. 



Mineral composition or mode. — The percentage of the actual 

 mineral components was determined according to the Rosiwal 

 method,'' by measuring with a micrometer the diameters of each 

 crystal in lines across the thin section. The feldspar, augite, olivine, 



« J. n. Pratt and J. V. Lewis, Corundum and the peridbtites of western North 

 Carolina, N. C. Geol. Survey, vol. 1, 1905, pp. 121-123. 



b Rosiwal, Vehr. Wien Geol. Reichs-Anst., vol. 32, 1898, p. 143 ff. See Cross, Iddings, 

 Pirsson, Washington, The quantitative classification of igneous rocks, 1903, p. 204. 



