Stone — Granitic Breccias of the Cripple Creek Region. 31 



It is well known that most volcanic regions show that at 

 times the lava flowed as a liquid over the surface, the flows 

 often alternating with layers of explosive ejectamenta, as at 

 Cap Rock a few miles S.W. of Cripple Creek. The internal 

 structure of the Cripple Creek volcanoes is difficult to make 

 out because the surface is so covered with debris. At the mar- 

 gins of a few of the larger lava bodies I have found signs of a 

 limited lateral flow, but in general the volcanic rocks occur as 

 dikes rather than as flows, and their motion was upward. 



The dikes numbered 2, 3, and 4 afford conclusive proof that 

 some of the dikes of the Cripple Creek region solidified at the 

 front before reaching the surface, also that they moved after 

 solidification. This gave the first definite clue to the mystery 

 of the granitic breccias. 



Dike 1 shows that the melted lava as it was pushed upward 

 ended at the front in a rather blunt liquid wedge which by 

 hydrostatic pressure successively wedged apart the rocks. This 

 probably could only happen near the surface where the rocks 

 were so broken they could thus be moved relative to each 

 other. 



We could naturally expect that as the frontal portion of the 

 melted lava cooled and became more viscous, this wedge would 

 become blunter. The time would come when the front of the 

 rising dike solidified or would become so viscous that the pres- 

 sure forward would become greater than the pressure tending 

 to wedge apart the solid rocks in front. If at this time the 

 pressure on the liquid from below was feeble or if the liquid 

 lava could force another vent, the movement might be arrested 

 at this point. If the upward movement continued, the only 

 way in which the dike could now rise was by pushing before it 

 a mass of the solid rock it was penetrating, crushing it into 

 fragments, and then as the fragments wedged each other apart 

 and ground against each other and the adjacent walls, it gradu- 

 ally rounded and polished them. The friction would be tre- 

 mendous and would fully account for the large amount of 

 crushed rock. If at this stage of the process the advance 

 should cease for awhile or become very slow, the heated waters 

 from below, where the lava was still liquid, would under the 

 proper conditions cement the crushed. granite into a firm sand- 

 rock while still between the granite walls of the dike, and 

 they might charge it with gold. If the motion were then 

 resumed the capping of sand-rock would be broken into blocks, 

 and as they were pushed onward they would become smoothed 

 and rounded, and would finally form a ridge rising above the 

 adjacent granite. 



For a time the liquid lava lay not far below the region of 

 crushed granite. In some cases, especially in case of the rhyo- 



