Stone — Granitic Breccias of the Cripple Creek Region. 27 



by moderate slopes. On the shores of the largest lakes in 

 Maine the stones of the unmodified till have not since glacial 

 time become so round as many of these cobbles and bowlderets 

 at the reservoir. One per cent or so of the pebbles and cob- 

 • bles show very round outlines, except on certain sides where 

 they have nearly flat surfaces of fracture but little modified. 

 This proves that many of the stones, after having become 

 round as eggs, were broken and the process of grinding was 

 then renewed, but had not gone far when the movement 

 ceased. Several of the rounder stones had been fractured at 

 each end, and in one case the two planes of fracture were 

 inclined to each other 30°. These fractured cobbles were of 

 massive granite, not of fissile schists. All this is just what a 

 dike could be expected to do if it pushed before it a mass of 

 fragments of the rock it was penetrating, they being confined 

 between solid walls. It would grind and roll and round them 

 and from time to time break them, and we would find every 

 stage from fresh fracture forms to the roundest all indiscrimi- 

 nately mixed as we do here. 



The difficulties of the lacustral theory are great and, as I 

 believe, insuperable. In dozens of places in the Cripple Creek 

 region the same sort of granitic grits occur underlain invari- 

 ably by volcanic dikes, and often underlain by breccias com- 

 posed largely of volcanic rock. We cannot trace any sharp 

 line of demarcation between these volcanic breccias and the 

 overlying granitic grits and bowlder beds. They pass into 

 each other by degrees ; the shapes of the fragments are the 

 same. We cannot postulate so many local lakes, and if we did 

 they cannot adequately account for the shapes of the frag- 

 ments or the invariable association with dikes. They occur on 

 the tops of hills as well as in valleys, and at such different 

 elevations they cannot be remains of a once continuous forma- 

 tion. A lake having a surf violent enough to roll and round 

 bowlders up to four feet in diameter ought to be able to exca- 

 vate a beach cliff and terrace and to plane off the rock to water 

 level. Nothing of the kind has been found anywhere in the 

 Cripple Creek region. On the contrary, these granitic grits 

 overlie steep slopes of erosion. On this point my observations 

 accord with Cross's descriptions. But in the case of dikes 

 numbered 3, 4, and 5, the same sort of rounded stones and 

 bowlders occur between granite walls and beneath the surface, 

 where there can have been no sedimentation by lakes or streams. 

 The lacustral theory is thus proved to be needless. We can 

 account for everything on the volcanic theory. 



7. Khyolite dike situated about one mile southeast of Mari- 

 gold. Elevation 8000 feet. 



This dike is situated on the top of the high ridge that lies 



