562 



REPORT OF NATIONAL MUSEUM, 1890. 



6. The diabases. 



Diabase, from the Greek word Siafiaffis, a passing over ; so called by 

 Bronguiart because the rock passes by insensible gradations into diorite. 

 Mineral composition. — The essential constituents of diabase are pla- 

 gioclase feldspar and augite, with nearly always magnetite and apatite 

 in microscopic proportions. The common accessories are hornblende, 

 black mica, olivine, enstatite, hypersthene, orthoclase,quartz,and titanic 

 iron. Calcite, chlorite, hornblende, and serpentine are common as pro- 

 ducts of alteration. Through a molecular change known as uralitiza- 

 tion the augite not infrequently becomes converted into hornblende, as 

 already described (page 516), and the rock thus passes over into diorite. 

 The plagioclase may be labradorite, oligoclase, or anorthite. 



Chemical composition. — The average chemical composition as given 

 by Zirkel is as follows : Silica, 49.54 ; alumina, 15.50 ; iron protoxide, 



14.27 ; lime, 8.20 ; magnesia, 

 5.29 ; potash, 1.1G ; soda, 3.3S ; 

 loss by ignition 2.29 per cent. 

 Specific gravity, 2.8. 



Structure. — In structure these 

 rocks are holo-crystalline gran- 

 ular, very rarely showing any 

 trace of amorphous or glassy 

 base.* Rarely do the constit- 

 uents possess perfect crystal 

 outlines, but are more or less 

 imperfect and distorted, owing 

 to mutual interference in proc- 

 ess of formation, the granular 

 hypidiomorphic structure of 

 Prof. Rosenbusch. The augite 

 in the typical forms occurs in 

 broad and sharply angular 

 plates inclosing the elongated or lath-shaped crystal of plagioclase, 

 giving rise to a structure known as ophitic (see Fig. 96). The rocks are, 

 as a rule, compact, fine, and homogeneous, though sometimes porphyritic 

 and rarely amygdaloidal. 



Colors. — The colors are somber, varying from greenish through dark- 

 gray to nearly black, the green color being due to a disseminated chlo- 

 ritic or serpentinous product resulting from the alteration of the augite 

 or olivine. 



Classification. — Two principal varieties are recognized, the distinction 

 being based upon the presence or absence of the mineral olivine. We 

 thus have: (1) Olivine diabase, or diabase containing olivine, and (2) 

 diabase proper, or diabase without olivine. 



* hidings discovered the presence of a glass base in the diabase of Orange Mountain, 

 New Jersey. American Journal of Science, May, 1888. 



Fig. 96. 

 Micro-structure of Diabase. 



(Weehawken, New Jersey.) 



