THE BASIC MASSIVE ROCKS, ETC. 445 



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 mem- 

 bers 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 gab- 

 bros have an augite devoid of the diallagic parting, while others 

 with many of the properties of diabase possess an augitic con- 

 stituent with the parting highly developed. " Hochstens 

 diirfen sie (the gabbros) als ein unterabtheilung der Diabase, 

 welche sich durch eine eigenthumliche Structur und Theil- 

 barkeit 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 princi- 

 pally 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 pinacoi- 

 dal parting, but was inclined to look upon them as rocks occu- 

 pying a position in the scheme of classification intermediate 

 between that of the diabases and that of the norites, the latter 



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 preexisting 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. 



