194 GEOLOGY OF THE COMSTOCK LODE. 



mitted to be of eruptive origin, must, I think, be sought in a period anterior to 

 the extrusion of the mass. The granular diorite is composed of crystals of 

 "secondaiy consolidation," interlocking grains, the relative position of which 

 cannot have changed subsequent to their formation. This rock must, there- 

 fore, have crystallized in its present position, barring, of course, aiiy move- 

 ments to which it may have been subjected after solidification. The porphy- 

 ries, on the other hand, are composed of well-developed crystals in a granu- 

 lar groundmass. These crystals must have grown .slowly in a magma suffi- 

 cientl}^ fluid to permit of free movement, and this condition is not likely to 

 have been present after eruption. A state of considerable fluidity is also 

 indicated by traces of brecciation in some of these rocks, and of fluidal 

 structure in the arrangement of microlites in a few slides. But the strong- 

 est evidence of a fluid condition is furnished by the little dike close to the 

 Eldorado croppings. The walls are granitoid, and the center of the dike is 

 semi-porphyritic, showing green fibrous hornblende and a granular structure, 

 though some porphyritical crystals are imbedded in it. But for an inch 

 from the walls of the dike the rock is a dark, solid porphyry which contains 

 brown hornblendes, and is in all respects similar to the most porphyritic 

 varieties found in the District. The contact with the walls is perfect, and 

 the occurrence admits of no natural explanation but that of a hot intrusive 

 fluid. 



Hypothesis suggested. — Tlic porpHyi'itical crystals formed before eruption 

 must have sunk to the bottom of the fluid mass, for the specific gravity of 

 hornblende is far greater than the mean density of the diorite, and the 

 relation can hardly have been reversed at the temperature at which they 

 formed. Little as we know of the subterranean conditions of eruption, it 

 is probably safe to assume that the upper portion of a fluid or plastic mass 

 would be extruded before the lower, and that the portion holding the por- 

 phyritical crystals in suspension would be the last to appear. The dike of 

 porphyry between granitoid walls already referred to seems to show that 

 this was the case, while the frequency of transitions is evidence that the 

 extrusion was a nearly continuous process. The granular groundmass of 

 the porphyries is finer-grained than the granitoid rock, but this does not 

 necessarily prove that it cooled under different conditions, for a certain dif- 



