55 



urged therefore against the adoption of this definition : (1) that it would 

 be impossible to recognise a granular rock without the examination of a large 

 series of microscopic preparations, and (2) it would so alter the recognised 

 limits of granular and porphyritic rocks as to introduce a large amount of 

 confusion the latter group would be enormously increased at the expense of 

 the former. 



In what follows a rock will be said to be granular when it is entirely or 

 almost entirely composed of crystalline constituents of uniform size, and 

 porphyritic when any of the crystalline constituents stand out in a marked 

 manner in consequence of size, and the extent to which crystalline form is 

 developed. 



Consider now the relation between the granular and porphyritic rocks. 

 The first fact that strikes one is that the granular texture is especially 

 characteristic of plutonic ; the porphyritic texture of volcanic masses. 

 Rhyolitic, andesitic and certain doleritic lavas exhibit the porphyritic textures 

 in the greatest perfection. Intrusive masses of granite, diorite, dolerite 

 (diabase) and gabbro, on the other hand, usually furnish good illustrations of 

 the granular texture. How is this fact to be accounted for ? The explanation 

 which naturally suggests itself is this. Lavas when they reach the surface 

 frequently contain crystals which have been developed before eruption, and 

 these crystals give the porphyritic texture after final consolidation. The 

 matter which is in the liquid state at the time of eruption consolidates as the 

 groundmass of the rock. If this view be accepted, and it certainly is 

 applicable to a large number of cases, we see that the porphyritic texture is 

 the expression of the fact that the conditions of consolidation changed 

 abruptly at the period of eruption. After crystalline growth had progressed 

 to a certain extent the semi-molten mass moved upward, and final consolida- 

 tion took place under different conditions. 



The porphyritic texture, however, is not only found in lavas but also 

 in veins, dykes and intrusive masses. The material of veins and dykes 

 merely represents lava which has failed to reach the surface, and therefore 

 the explanation given above will apply to these cases with only a slight 

 modification. 



The relations between the granular and porphyritic textures in intrusive 

 masses is of great interest. In many cases a perfectly gradual transition 

 from one texture to the other may be observed ; the central portions 

 of a mass frequently showing a granular texture, while the marginal portions 

 and the apophyses are more or less porphyritic. Thus a dyke in the Hartz, 

 known as the Bode-Gang (1) which is connected with the granite-mass of the 

 Rammberg is a true quartz-porphyry, containing in places an amorphous base 

 and quartz-crystals with a few liquid- and many glass-inclusions. A some- 

 what similar dyke occurs at Mt. Willard in New Hampshire. (2) It is an 

 off-shoot from the main mass of Albany granite. The central portions are 

 holocrystalline and the texture is thoroughly granular. The quartz occurs 



(1) LOSSEN. Der Bode-Gang im Hartz. Z.D.G.G., 1874, XXVI., 856. 



(2) HAWKS. The Albany Granite and its contact phenomena. A. J.S. XXI. p. 21, Jan. 1881. 



