8 PROCEEDINGS OF THE NATIONAL MUSEUM vol.71 



red and green and other tones. Induration by hornfels formation 

 causes the sediments to resist erosion almost as much as does the 

 quartz monzonite, and where the former have been somewhat bleaclied 

 it is often no easy matter to detect the actual contact except by close 

 examination. Teocalli Mountain (13,220 feet) exhibits a striking 

 example of the change in color of beds in the contact zone. 



One commonly observable feature of the alteration of the limestone 

 pebbles wliich are abundant in the Maroon conglomerate is the 

 change of the mass of the pebble into white granular marble, while 

 the impurities have been concentrated in one or at least only a few 

 good-sized crystals of garnet. 



In some parts of the great intrusive body of quartz monzonite 

 there are many huge xenoliths of sedimentary beds torn loose from 

 the irregular walls. Some 16 of these were large enough to indicate 

 on the Crested Butte map. Such bodies are naturally very greatly 

 altered, much lilve the contact zone rocks. Few of them were acces- 

 sible for close examination. 



The quartz monzonite intrusives have suffered very little from 

 alteration of any kind. But they were considerably and irregularly 

 fissured at an early date, and in these fissures, seldom more than an 

 inch or two in diameter, there was deposited a more or less complete 

 filling of one or more of the secondary minerals characteristic of 

 the contact zone. In some places an aplitic or pegmatitic filling is 

 complete. Mr. Shannon has examined the specimens collected and 

 finds the species described in the following pages generally occur- 

 ring in the wall linings where cracks have been but partially filled. 



In conclusion, there are several special points of interest attaching 

 to this district of contact zone mineralization which may be sum- 

 marized. The minerals are mainly silicates such as are commonly 

 produced in contact zones where a sedimentary section of various 

 rocks, largely carbonates, is penetrated by extensive igneous bodies. 

 The reactions resulting in the new compounds arise from the per- 

 meation of the sediments under a condition of high heat, long sus- 

 tained, by hot aqueous solutions carrying absorbed gases and other 

 " mineralizing agents." 



The particular interest of this occurrence lies partly in the un- 

 usual perfection of development of some species, as vesuvianite, gar- 

 nets, pyroxenes, and amphiboles. The specimens obtained Avere 

 secured in a short time in the course of geological field work. It is 

 evident that when carefully explored as a mineral locality a much 

 more extensive collection and doubtless of finer quality can be made. 



Another feature of note is that while the intrusive masses are very 

 similar petrographically to many others known in the adjacent 

 country to the south or west there is no corresponding contact zone 



