56 



as irregular grains without any approach to crystalline form. At a distance 

 of fifteen feet from the junction the rock consists of crystals of orthoclase 

 and bipyramidal quartz embedded in a matrix which cannot be resolved 

 into distinct constituents by the naked eye. At the actual junction the 

 matrix becomes hornstone-like in appearance. Many similar illustrations 

 of the relation of granite to quartz-porphyry might be quoted. 



In the intermediate and basic groups we find similar relations between 

 the granular and porphyritic textures in large masses. The rocks of the 

 Washoe district in North America may be quoted as an illustration of this 

 statement so far as the intermediate group is concerned. This case is of 

 considerable importance because the rocks have been well exposed in con- 

 sequence of the extensive mining operations connected with the Comstock 

 Lode. Moreover the facts have been described by a number of competent 

 observers. (1) The rocks involved are pyroxene-andesite, hornblende-andesite, 

 diabase and diorite. The pyroxene- and hornblende-andesites are typical 

 porphyritic rocks, containing more or less amorphous matter in the ground- 

 mass. The diabase and diorite are holocrystalline rocks, usually porphyritic, 

 but sometimes perfectly granular. The earlier observers, RICHTHOFEN, 

 ZIRKEL and BECKER, acting under the impression that the granular texture 

 and other characters possessed by the diabase and diorite were especially 

 characteristic of pre-tertiary rocks, separated these rocks from the pyroxene- 

 and hornblende-andesites which they considered to be of Tertiary age. 

 Messrs. HAGUE and IDDINGS have recently shown that the diabase and 

 pyroxene-andesite are merely textural modifications of the same rock-mass ; 

 and that the same is true of the diorite and hornblende-andesite. Diabase 

 shades into pyroxene-andesite ; diorite into hornblende-andesite. The 

 granular texture is developed in the greatest perfection about Mt. Davidson. 

 In proceeding outwards from this mountain it gradually gives place- to the 

 porphyritic texture. 



All these facts point to the conclusion that the texture of a rock is 

 determined by the conditions of consolidation; and the most reasonable 

 supposition is that the coarse granular texture is characteristic of slow 

 consolidation under considerable pressure and uniformly changing conditions, 

 while the porphyritic texture is produced by one or more abrupt changes in 

 the environment of the consolidating magma. 



In igneous rocks which retain their original characters it is possible to 

 determine, in most cases, the order of crystallisation, by observing the extent to 

 which crystalline form is developed and the various ways in which the different 

 minerals interfere with and enclose each other. In some of the typical 

 granular rocks, however, this becomes impossible. The principal mineral 

 constituents mutually interfere with each other as if they had crystallised 

 simultaneously. Now it is interesting to note that this latter feature is the 



(1) RICHTHOFEN. The Comstock Lode, San Francisco, 1868. 



ZIRKEL. Microscopical Petrography, U.S. Geol. Expl., 40th Parallel, 1870. 

 BECKER, G. F., Geology of the Comstock Lode. 



HAGUE AND IDDINGS. On the development of crystallisation in the igneous rocks of 

 the Washoe District. Bulletin of the U.S. Geol. Survey, No. 17, 1885. 



