PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 157 



Diabase. Laumontite and caleite.l 



of Bryn Mawr College, who has carefully studied these South Mountain rocks, and she 

 states that it is impossible to reach any other conclusion regarding the rock in ques- 

 tion (No. 68) than that it is an ancient acid lava. She also states that the thin 

 seams of quartz are probably cross sections of the former layers of spherulites.* 



u. s. G. 

 No. 69. DIABASE. 



Near London, forming a point next east of the niouth of the creek, and not more than six rods from it; 

 by its position apparently overlying the last, cut by a dike, north 5 east, three feet wide. 

 Ref. Annual Report, ix, page '21 . 



A brown, compact, medium grained, scantily porphyritic rock, with 

 epidotic linings in the seams. 



Mii: The poor slide at hand only shows that this is, an ordinary diabase, with 

 feldspathic microlites trachytically exhibited. The augite areas are converted to a 

 greenish, probably chloritic substance, and the presence of original olivine can only 

 be predicated on the apparently independent outlines of some of these greenish areas. 

 In general the minerals are all reddened by hematite. 



. l//r. Cabotian. N. H. w. 



No. (3!) A. LAUMONTITE AND CALCITE. 



On the east of the dike cutting No. 69 this rock is roughly amygdaloidal, but subsequently resumes its 

 dense structure and thus continues more than half way to Lester river, cut by occasional dikes. 

 Ref. Annual Report, ix, page 21. 



With a layer of calcite, next the rock, a second growth of a brick-red 

 mineral is deposited on the calcite. The latter is closely fibro-lamellar and fragile, 

 but less so than is usual for laumontite. 



Mir. This red mineral gives gelatinous silica in boiling HC1, has low refrac- 

 tion and low double refraction. Its cleavage plates, when examined in fine powder, 

 frequently give an oblique optic axis, and it has n p in the acute optic angle, making it 

 negative. Its angle of extinction with the principal elongation of the fibres is large. 



These characters combined indicate laumontite. Its somewhat greater dura- 

 bility and darker red color are to be attributed to a copious cementation by infiltrated 

 iron oxide. The sign of the elongation is positive, but as the angle of extinction 

 measured on the elongation is about 25, it is necessary to make that allowance for 

 the position of the axis of elasticity.f 



Cabotian. N. H. w. 



*Cf. an illustration of aporhyolyte with spherulites in layers. F. Bascora: The ancient volcanic rocks of South Moun- 

 tain, Pennsylvania. U. S. Geol. Survey, Hulli-lin ca < ' ' '. pi. 10, 1896. 



t Hull, tincifle i/i' .Ifiiiei-iiloiiii' de France, vol. viii, 322. The student will find here a useful classification of all zeolitic min- 

 erals according to their optic characters, followed by a tabulation of their diagnostic characters. The zeolites having negative 

 elongation arc those in which the smaller index of refraction (HP, o) is parallel, or nearest parallel, with their greatest dimension, 

 a> determined by the easy cleavages. The direction of this axis is easily observed by crushing fine a small quantity of the mineral 

 and observing the colors between crossed nicols with the quartz plate interposed when the mineral and the quartz plate have 

 their axes at -15 from the principal directions of the nicols. There is a later and improved classification of the zeolites in Lacroix's 

 Miiitriilniiii- ill' In i'rnn,',' ,1 ill- ie I'ol'ini.'x, vol. ii, 1897. In the case of laumontite the axis n g is nearer parallel with the principal 

 cleavages and the elongation is normally positive. 



