686 GEOLOGICAL SUEVEY OF THE TEERITOEIES. 



schists, gneisses, and granites, all occur; the gneiss, with possibly gran- 

 ite, in the greater proportion. While large areas of structureless gran- 

 ite abound, apparently of so-called plutonic or eruptive origin, search 

 seldom fails in finding spots or areas more or less large of gneissic or 

 even distinct schistose structure. The fact that these usually merge im- 

 perceptibly into the surrounding granite, as well as conform in their 

 strikes and dips to the general system of folds, as more plainly indicated 

 perhaps in adjacent schistose regions, show that such granites have 

 been metamophosed in situ, and are indigenous rocks. At the same 

 time, sharp lines of demarkation, and the occurrence of dikes and allied 

 features, show that the conditions of extreme metamorphism have prob- 

 ably been accompanied by a great softening of the rock, allowing ready 

 molecular re-arrangement into structureless forms, and producing plu- 

 tonic and other appearances indicative of an exotic character. 



The same granite mass, approached from opposite sides, might convey 

 entirely different impressions as to its origin ; a metamorphic indigenous 

 nature being indicated upon the one hand ; an eruptive, exotic origin 

 upon the other. 



I doubt if any of the large granite masses of the mountains are of true 

 intrusive character, and even if those smaller ones which are clearly 

 intrusive have come from great distances below, or are other than of the 

 same series of rocks melted by the heat accompanying the metamorphism 

 of the mass. 



Along the south side of the map, and exposed by the canon of the 

 Boulder Creek, are massive gray granites, with but few points where 

 any structure was observed. 



All along this half of the map, the general strike is approximately east 

 and west, with a northern dip. This is the case also along its west border. 

 IsTear the north and east sides, however, the dip is south, indicating a 

 synclinal structure running through the middle of the eastern portion of 

 the map. 



A horizon in which a definite schistose structure tends to occur is in- 

 dicated by the dotted area running through the center of the map. Some 

 of the rocks here are distinct schists, but little changed, and include very 

 irregular red and black banded mica-schists, garnetiferous, and some 

 handsome, fine, and evenly-banded, gray gneissic schists. In places, 

 this zone may be lost in granite, but opportunity did not offer to care- 

 fully follow it throughout. ^ 



Most of the granite on the north edge of the map contains little if any 

 mica, tending to a reddish granular aplyte. The schist zone shows the 

 fold of the formation very well, some of them being very abrupt, and 

 regions of great contortion. All the observed strikes and dips are in- 

 dicated on the map, but the general structure of so small an area cannot 

 be well shown separated from the surrounding country. 



These schists and granites are pierced at many points by a number of 

 dikes of felsite porphyry, which are also indicated on the map. Usually 

 these form hills or ridges, and while some are quite long, the porphyry 

 has apparently often found vent through less extended openings, now 

 showing as sugar-loaf-formed hills, without the direction of the dike be- 

 ing clearly indicated. Such forms are shown by a cross. The porphy- 

 ries vary considerably in character, but no careful comparative examin- 

 ation of them has yet been made. Some contain remarkably handsome 

 crystals of feldspar, often of the form of the Carlsbad twins. 



The tellurium ores of Gold Hill occur in connection with one of these 

 dikes, (see section beyond.) This dike varies from 45 to 35 feet in width, 

 trends about north 30° east, and dips approximately 80° to the north- 



