266 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



developed along the planes of schistosity. (Plate 27, Fig. B.) The general 

 ground-mass of the rock 13, as a rule, a light to medium-tinted greenish-gray, 

 silvery, glittering felt of quartz and abundant sericite. Sprinkled through the 

 felt are the round or hexagonal biotite plates, which range from 1 mm. to 3 mm. 

 in diameter. The biotite is highly lustrous, and, on account of its darker 

 colour, stands out prominently on the surface of the rock. This special pseudo- 

 phenocrystic development of biotite i9 characteristic of the whole belt and, 

 while occasionally seen in narrow bands of belt D, is not an essential feature 

 of any other than belt E. For this reason it may be called the belt of ' spangled 

 schists.' 



Along with the biotite spangles there are often many pale-reddish anhedral 

 garnets also developed in the planes of schistosity. The sericite is commonly 

 replaced by well characterized muscovite of the ordinary type, though it never 

 takes on the size of the biotite spangles. Around the large biotites and the 

 garnets the small shreds of sericite and quartz grains are often seen under the 

 microscope to be arranged in concentric layers; this relation is the familiar 

 one to be observed so often in garnetiferous schists. A little magnetite, a few 

 zircons and needles of rutile form the accessories of the schist. 



There are all stages of transition between the typical spangled schist and 

 sheared quartzite, which is always sericitic and commonly speckled with minute 

 dots of dark biotite. These quartzites are similar to those characteristic of 

 both belt D and belt F. 



Near the divide between Summit creek and the north fork of Corn creek 

 the spangled schists enclose a band of common amphibolite about one hundred 

 feet in apparent thickness. This is evidently a sheared and highly metamor- 

 phosed basic igneous rock, probably of intrusive origin. A second sill-like 

 intrusion of much altered basic rock (now a hornblende-chlorite schist), with 

 an exposed width of ten feet, was found on the ridge about a mile and a half 

 E. N. E. of North Star mountain. With these exceptions belt E is a fairly 

 homogeneous body of acid, sedimentary rock wholly metamorphosed. 



The schistosity planes usually strike parallel to the boundaries of the belt 

 as laid down on the map; the dip is always very high, varying from 90° to 

 75° E. It is apparently more characteristic of this belt than of any of the 

 others that the attitude of the bedding is highly discordant with that of the 

 schistosity. The two planes were often seen, in the same ledge, to cut each 

 other at angles of from 60 to 80 degrees. Unfortunately the exposures were 

 not sufficiently numerous to enable the writer to determine even the main facts 

 concerning the true position of the bedding planes throughout the belt. It is 

 only known that these rocks are often greatly crumpled and that the folds 

 and crinkles are crossed indifferently by the master-structure. Considering tho 

 intense metamorphism, the bedding is well preserved and is represented by 

 good contrasts of colour between the lighter tinted, more quartzitic layers and 

 the darker, more micaceous layers once rich in argillaceous material. (Plate 

 27, B). 



The spangled schists were followed from the Boundary line to the ridge 



