452 THE JOURNAL OF GEOLOGY, 



gabbro to granitic forms of plagioclase pyroxene rocks, and to 

 designate as diabases the ophitic, porphyritic and glassy forms. 

 He agrees with Zirkel 1 and Lasaulx 2 in regarding the Hebridean 

 rocks as Tertiary in age, and at the same time as corresponding 

 in all their characteristic features with older augite-plagioclase 

 rocks of granitic structure. These rocks possess not only the 

 structure of the most typical gabbros, but their various constitu- 

 ents are marked by the same microstructure. The plagioclase, 

 olivine, and augite contain the numerous inclusions that were so 

 early recognized as characteristic of these minerals in gabbro, 

 and the latter mineral, the augite, is marked by the diallagic 

 parting, which is the result of the action of a secondary process 

 upon ordinary augite. The process, called by Professor Judd 3 

 schillerization, is moreover shown to be a function of the depth 

 at which the original rock magma cooled, and the granitic struct- 

 ure of the rock mass is demonstrated to be likewise due to the 

 fact that the rock possessing this structure crystallized at some 

 depth below the earth's surface. 



The work of Professor Judd established two great facts, viz.: 

 first, that the age of a rock cannot serve as a basis for rock 

 classification, since it has but little to do with the development 

 of a characteristic structure ; and, second, that the geological 

 position of a rock mass is the condition determining not only its 

 structure, but also the peculiar features possessed by its constit- 

 uents. The rocks which it is proposed to call gabbros are 

 marked by both of the characteristics of deep-seated rocks, 

 while the diabases possess neither of them. The differences 

 between the two groups of rocks, as expressed by their structures, 

 are probably differences that are dependent upon the geological 

 conditions under which they solidified. 



Zeits. d. deutsch. Geol. Gesell. XXIII, 1871, pp. 58 and 93. 



2 Min. u. Petrog. Mitth. I, 1878, p. 426. 



3 Cf. also J. W. Judd : On the Relations between the Solution-planes of Crys- 

 tals and those of Secondary Twinning ; and on the Mode of Development of Negative 

 Crystals along the former. A Contribution to the Theory of Schillerization. Min- 

 eralog. Magazine, VII, p. 81. 



