. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. THE PEEIBOTITES 91. Structure. — The structure as displayed in the different varie- ties is somewhat variable. In the dunite it is as a rule even crystalline granular, none of the olivines showing perfect crystal outlines. In the picrites the augite or hornblende often occurs in the form of bx^oad plates occupying the interstices of the olivines and wholly or partially enclosing them, as in the hornblende picrite of Stony Point, New York. The saxonites and Iherzolites often show a marked porphyritic structure produced by the
. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. THE PEEIBOTITES 91. Structure. — The structure as displayed in the different varie- ties is somewhat variable. In the dunite it is as a rule even crystalline granular, none of the olivines showing perfect crystal outlines. In the picrites the augite or hornblende often occurs in the form of bx^oad plates occupying the interstices of the olivines and wholly or partially enclosing them, as in the hornblende picrite of Stony Point, New York. The saxonites and Iherzolites often show a marked porphyritic structure produced by the development of large pyroxene crystals in the fine and evenly granular ground-mass of olivines. (See Fig. 5, as drawn hv Dr Cr TT Wil k/j x^x. \x. XX. vv ii- pjq^ 5^—Mierostructure of porphyritic Iherzo liams.) Ihe rocks belong ixte, partly altered into serpentine. to the class designated as hypidiomorphic granular by Professor Eosenbusch; that is, rocks composed only in part of minerals showing crystal faces peculiar to their species. Colors. — The prevailing colors are green, greenish gray, yel- lowish green, dark green to black. Nomenclature and Classification. — Mineralogieally and geo- logically it will be observed the peridotites bear a close resem- blance to the olivine diabases and gabbros, from which they differ only in the absence of feldspars. Indeed, Professor Judd has shown that the gabbros and diabase both, in places, pass by insensible gradations into peridotites through a gradual dimi- nution in the amount of their feldspathic constituents. Dr. Wadsworth would extend the term peridotite to include rocks of the same composition, but of meteoric as well as terrestrial origin, the condition of the included iron, whether metallic or as an oxide, being considered by him as non-essential, since native iron is also found occasionally in terrestrial rocks, as the Greenland basalts and some Please note that these images are extracted from scanned page
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