THE EASTEKN AIIEA. 



413 



Fia. 11. — Hornhlt'nd*- enlargement of - 

 au;ritc iu diabase. 



defined, broken right lines, or lines somewliat curved, as tiie spaces left by 

 the feldspars allowed. Often the new growth has continued farther in places 

 than in adjoining ones, and, as it went on, it lias sonietiiiK's widened out, 

 forming club-shaped protuberances of hornblende within the feldspar, in 

 longitudinal sections (Fig. 10) the sinaragdite is more plentiful at the 

 extremities of the individuals than at the sides. The terminations of these 

 enlargements are sharply serrate. The constituent fibers at times are 

 slightly divergent and usually cut deejdy into 

 the feldspar. Ordinarily the fine fibrous cleav- 

 age of the smaragdite is coincident with the 

 cleavage of the augite. However, the angles 

 C : C in the augite and in the enveloping smarag- 

 dite show their characteristic relations. In trans- 

 verse sections (Fig. 11), where the intersecting 

 prismatic cleavages of the augite and smaragdite 

 are seen, then' relations are such as to indicate 

 that the oxtho- and clino-pinacoids iu the two minerals are parallel. The 

 crystallographic relations of the two minerals ai-e, then, in both longitudi- 

 nal and transverse sections, precisely like those well known to occur be- 

 tween augite and amphibole paramorphic after it.' 



Another difference betweeuthe diabases of the Western and Eastern 

 areas is the relative freshness of the pyroxene as compared with the plagio- 

 dase in the latter. Generally pyroxene decomposes more rapidly tluui 

 plagioclase. The reverse has been the case here. In nearly every section 

 the plagioclases have been found to be quite extensively altered. The 

 double angles in the zone 001 : 100 do not exceed 62°, which would indi- 

 cate that the most basic feldspar is labradorite. The most frequent altera- 

 tion jTi-oducts of the feldspar are small brilliantl}- polarizing flakes, taken to 

 be kaolinite, green smaragdite, chlorite, epidote, biotite, and cpiartz. In the 



• Teall, J. J. H. : Quart. Jour. Geol. Soc, London, vol. 40, p. 353, PI. 29. Fig. 3. Eohrbacli, Carl 

 E.*M.: Uobcr die Eruptivgesteiue iiu Gebiete dor schlcsisfU-miihrischun Krcidi'ibrmation; Sliii. ii. 

 pctrog. Mitth., vol. vii, p. 24, PI. 1, Fig.s. 1-7. Van Hi.sc, C. K. : Note ou the enlargement of horn- 

 blendes and angitcs in fragmental and eruptive rocks; Am. .Jour. .Sci., 3d series, vol. 33, 1887, p. 385. 

 Holtlis, Wm. 11. : On tlie iictrograpUical iharaeteis of a dike of diabase iu the liostou basin ; Hull. Mus. 

 Coiup. Zool. Harvard Coll,, vol. IG, 1887, p. 10, PI. 1, Fig. 2. 



