372 Ball — Pre- Cambrian Hooks of Georgetown, Col. 



ridges, however, existed where sharp peaks now are, and the 

 valleys between were broader and less steep than those of the 

 present streams. The drainage was dendritic and mature, the 

 surface being adjusted to the structure of the underlying rocks 

 and to the varying resistances to erosion of the different for- 

 mations. Lakes did not then exist. The period at which this 

 mountainous surface reached maturity is unknown, but it was 

 probably in late Tertiary or early Pleistocene time. 



After the old. upland had been formed, deformation increased 

 the gradient of the streams and they cut the present canyons, 

 straight-walled and beset with pinnacles and rugged ribs of 

 rock, in the old broad valleys. The valley-heads of these 

 revived streams were afterward occupied by alpine glaciers of 

 two distinct epochs. The glaciers eroded cirques, aretes, 

 U-shaped valleys, hanging valleys and lake basins, and by the 

 deposition of lateral, terminal and. ground moraines further 

 modified the topography of the upper portions of the valleys. 



General Geology. 



Preliminary outline. — The rocks of the Georgetown quad- 

 rangle, with the exception of Pleistocene deposits and intru- 

 sive ignedus rocks possibly of Tertiary age, belong to the pre- 

 Cambrian complex of the Colorado Range. The oldest rocks, 

 named in this article the Idaho Springs formation, are crystal- 

 lines probably of sedimentary origin. These rocks have been 

 most intricately injected by a series of holocrystalline igneous 

 rocks, presumbly of pre-Cambrian age. So intense is injection 

 that the rocks of the quadrangle may be considered an immense 

 igneous breccia, exposure after exposure being encountered in 

 which it is difficult to decide whether to map it as the older rock 

 intruded by the younger, or as the younger rock with inclusions 

 of the older. Some idea of the complexity of injection may be 

 gained from the fact that in a distance of one mile, on the ridge 

 between Silver Creek and Clear Creek, six formations alternate 

 seventy-six times, or at the rate of one alternation to 70 feet. 

 This is exclusive of a number of minor injections and inclu- 

 sions. 



That the injections of the pre-Cambrian igneous rocks took 

 place at widely separated periods is shown by the different 

 degrees of schistosity developed in the different formations. 

 The granitoid habit of the igneous rocks and the character of 

 their metamorphism indicate that the present surface during 

 the whole period of pre-Cambrian intrusion was buried beneath 

 deep masses of overlying rocks. 



At three points near Chicago Creek, at elevations of from 

 9,200 to 10,100 feet, residual bowlders of red or brown silicified 

 sandstone are rather abundant. Pebbles of the pre-Cambrian 

 granites are contained. Outcrops of lithologically similar 



