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THE BASIC MASSIVE ROCKS, ETC.

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regarded as sub-groups of the gabbro family, but were looked upon merely as altered gabbros. Magnetite and titanic iron oxide as well as apatite were mentioned as accessory in all members of the group, and hornblende, rhombic pyroxene, brown mica and quartz were spoken of as occurring in many (p. 459). The difficulty of distinguishing between a gabbro and a diabase was clearly appreciated. The distinction between diallage and augite, upon which is based the mineralogical distinction between gabbro and diabase, is acknowledged to be of doubtful value for this purpose, since some rocks with the other properties of gabbros have an augite devoid of the diallagic parting, while others with many of the properties of diabase possess an augitic constituent with the parting highly developed. "Höchstens dürfen sie (the gabbros) als ein unterabtheilung der Diabase, welche sich durch eine eigenthümliche Structur und Theilbarkeit ihres Pyroxens charakterisirem." The structure of the gabbros was said to vary within narrow limits. They are always coarse-grained rocks whose different structures depend principally upon the different amounts of their constituents. Since they are so well characterized by the monotony of their texture, and since no gradations^[1] between them and porphyritic or glassy forms were known, while on the other hand the structure of the diabases varies so widely between holocrystalline and glassy, the former were regarded as a distinct rock type. Rosenbusch, however, declined to regard the gabbros as dependent for their individuality upon the mere possession of an augite with pinacoidal parting, but was inclined to look upon them as rocks occupying a position in the scheme of classification intermediate between that of the diabases and that of the norites, the latter

↑ Mr. T. T. Groom has recently described a gabbro glass associated with gabbro at Carrock Hill in the Lake District, England, under the name carrockite. Since this glass occurs only as a narrow selvage where the gabbro has cooled rapidly in contact with preëxisting rocks, it cannot be considered as contradicting the above general statement. The structure is not one connected genetically with the rock itself, but is a local phenomenon dependent upon extraneous circumstances. See T. T. Groom: On the Occurrence of a new form of Tachylyte in Association with the Gabbro of Carrock Fell, in the Lake District.Geol. Magazine. Jan. 1859, p. 43.

[1] Mr. T. T. Groom has recently described a gabbro glass associated with gabbro at Carrock Hill in the Lake District, England, under the name carrockite. Since this glass occurs only as a narrow selvage where the gabbro has cooled rapidly in contact with preëxisting rocks, it cannot be considered as contradicting the above general statement. The structure is not one connected genetically with the rock itself, but is a local phenomenon dependent upon extraneous circumstances. See T. T. Groom: On the Occurrence of a new form of Tachylyte in Association with the Gabbro of Carrock Fell, in the Lake District.Geol. Magazine. Jan. 1859, p. 43.

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