AREAL GEOLOGY OF LOWER CLEAR CREEK 271 



Fundamental Gneiss. Like the latter, its dip is prevailingly to the north. It is predomi- 

 nantly the home of the latite porphyry dikes. 



Microscopic. — The hand specimen consists of an evenly banded arrangement of quartz, 

 feldspar, and mica, but without the plications usual in the Fundamental Gneiss. 



Microscopic. — Under the microscope the rock is seen to consist of quartz, orthoclase, 

 plagioclase, microcline, biotite, muscovite, magnetite, apatite, epidote, titanite, calcite, 

 zircon, fibrohte, and hematite. Sericite is occasionally developed. 



The quartz is generally in excess and shows in every case undulatory extinction. The 

 feldspars appear as in the Fundamental Gneiss series, but in most cases show marked 

 undulatory extinction. The muscovite and biotite occur as independent aggregates, and 

 also frequently intergrown. The rock in question is distinguished from the Fundamental 

 Gneiss series by its greater resemblance to a normal granite and by the smaller quantity 

 of biotite. 



For the areas covered by the Idaho Springs and Central City special maps no attempt 

 has been made to separate the Fundamental Gneiss from the Clear Creek Gneiss on the 

 map. There are two reasons for this. 



In the first place, the two units are very intricately combined in one vast complex, 

 which has been subjected to repeated fissuring and faulting. The faulting is not impor- 

 tant, so far as the distance of movement is concerned, but when one considers the area 

 affected, it assumes considerable importance. Then, too, this special area has been almost 

 universally injected with pegmatite, and intruded by recent eruptive dikes. In addition 

 to all this, we have the metamorphism of the rock by the subsequent, or usually subsequent, 

 vein-filling solutions. All this causes a complex exceedingly difficult to unravel, and still 

 more difficult to represent on a small scale map such as must be used in a report of this 

 kind. It may be stated, however, that detailed notes have been taken for this area, and 

 material for a reasonably exact map is at hand. 



To anyone interested in the study of these Gneisses the Newhouse Tunnel offers an 

 admirable section of some three miles in length. The direction of the tunnel in round 

 niunbers is about N. 15° W. At a few hundred feet south of the Gem vein the tunnel 

 leaves the Fundamental Gneiss and cuts the Clear Creek Gneiss. This contact is still 

 further to the south on the surface; in other words, the contact dips to the north at an angle 

 of something hke 45°. While some porphyry dikes have been cut in the Fundamental 

 Gneiss, a far greater number are to the north in the Clear Creek Gneiss. The Funda- 

 mental Gneiss has been greatly plicated and more disturbed, it is true, than the Clear 

 Creek Gneiss to the north, but it is in this latter that we can see most clearly, though not 

 on such a large scale, the effects of dynamic action. This is fortunate, for in the narrow 

 section cut by the Newhouse Tunnel every possible example of faulting and folding is in 

 evidence in the course of a few thousand feet. These evidences of movement are rendered 

 clear by the frequent alternating bands of black diorite and white gneiss, and strongly 

 reminds one of the diagrams illustrating works on geology. In most cases the faults are 

 not composed of one simple fault-plane, but are a series of distributive faults or fault zone. 

 Frequently the fault-planes are so numerous and close together that we have simply a 

 zone of sheeting. One of the very interesting occurrences is that shown on the west side 

 of the tunnel just beyond the Sun and Moon vein (Fig. 2). 



