94 GEOLOGY AND MINING INDUSTRY OF LBADVILLE. 



as fell under observation, without pretending' to present them in any deter- 

 mined degree of completeness. The special study of the Archean rocks 

 in the tield was assigned to Assistant Whitman Cross, to whom also was al- 

 lotted the duty of examining them microscopically, and the greater part of 

 the observations here recorded are derived from his notes. Granites and 

 gneisses with accessory occurrences of amphibolite constitute, as already 

 stated in Chapter III, the main components of the Archean in j\[osquito 

 Range. As seen from one of the commanding peaks of the range the 

 most striking features of the rocks are the great irregular vein-like masses 

 of white pegmatite, which form an infinitely intricate network on a 

 background of darker gneiss. When examined more closely, however, 

 the definite outline of these pegmatite bodies is no longer so apparent, and 

 they are found to be intergrown in the surrounding rocks in a most intri- 

 cate manner. It is only in the smaller veins, such as are shown in Plate 

 IV, that their outlines can be definitely traced. Structure lines, as defined 

 by relics of former stratification, are so seldom to be distinctly traced that 

 no attempt has been made to co-ordinate the few facts observed into any 

 •general structural system. 



Of eruptive rocks in the form of dikes and intrusive masses of irregular 

 shape an almost infinite variety, both in form and composition, is found. 

 The dikes are generally naiTOw, being rarely over .50 feet in width, and of 

 limited continuous length. Those shown on the map are only the more 

 prominent of those actually observed, and it must be borne in mind that a 

 great portion jn-obably did not come under observation at all. 



NORTHEASTERN DIVISION. 



Platte amphitheater. — Like tlic Arkansas River, whose amphitheater adjoins 

 this on the west, separated only by a single narrow, knife-like ridge, the 

 Platte at its source flows first north and then bends round upon itself to 

 take its main course in a diametrically opposite direction. A reason for 

 this bv no means uncommon occui-rence in the glaciated regions of the 

 Rocky Mountains may be found in the fact that on tlie northern sides of 

 the higher peaks are the greatest and most permanent accumulations of 

 neve ice, to whose erosive action, not yet thoroughly studied, are doubtless 



