ENDLicH.] BASIC PLUTONIC EEUPTIVES. 209 



oligoclase, joined together rather loosely, without the slightest intimation 

 of a magma." * No definite relation, as borne by these dikes to the 

 surrounding metamorphic or prozoic rocks, could be observed. Without 

 rule they are distributed, striking in a definite direction within very cir- 

 cumscribed areas, but in no further connection with each other. Proba- 

 bly they are dikes that were formed catogeneously at the tiuie of meta- 

 morphosis. Their mineralogical composition is identical with that 

 usually accepted for diorite, and I therefore cite them as such. Similar 

 in many characteristics to this is the following species : 



Euphotide. — In the vicinity of Mount Ouray this rock occurs. It is 

 composed t of a dark-green diallage and white oligoclase. Chlorite and 

 magnetite are accessories. More important than the diorite, thie rock 

 builds up a considerable portion of the Ouray group. It is intimately 

 associated with the granite, which there appears to be the younger of 

 the two. In determining the specific position of any of the hornblendic 

 rocks within a metamorphic area the greatest caution is requisite. There 

 is but little doubt that the same hornblendic rocks that we now can 

 claim to be metamorphic, may have been, from one or the other cause, 

 eruptive. In that case they will not necessarily present any distinguish- 

 able mineralogical features, but the method of their association, the mode 

 of occurrence, must furnish the decisive evidence. Whenever this cannot 

 be obtained from such characteristics, the identification remains ques- 

 tionable. We may have before us instead of an erupted rock, simply 

 the metamorphosed product of one of the earliest sedimentaries. Un- 

 less, therefore, much labor and time should be spent upon this class of 

 rocks in Colorado, I cannot conceive that any satisfactory results will 

 be obtained. At best their horizontal distribution is but limited; in 

 their detail features they show so many variations, however, that the 

 most elaborate work will be required before they can all be assigned to 

 their proper places. I have chosen the examples of diorite and eupho- 

 tide as the best determined out of a number. Among it such rocks 

 as gabbro, hypersthene, and others might be mentioned. 



Porphyry. — Very few occurrences of porphyry in Colorado have been 

 noted. The most important and best known porphyry dike of the 

 State is that at Gold Hill.l On either side of this dike, which traverses 

 granite, is located a remunerative gold mine. Several smaller dikes are 

 found within a short distance of this one, but are of minor importance. 

 Great interest attaches itself to this occurrence from the presence of 

 tellurium compounds in the metalliferous veins adjoining the porphyry. 

 A purpli.sh-gray paste, varying to greenish, incloses very fine crystals of 

 feldspar. Orthoclase, with laminse of oligoclase, and small crystals, 

 apparently of microclinite, are the inclosed minerals. Another por- 

 l>hyry dike was observed in the Sangre deCristo Mountains.§ This has 

 broken through Carboniferous strata, and now appears near the summit 

 of a high peak near the Crestones. A dense, pink matrix incloses small 

 crystals of feldspar and dihexahedral quartz. 



Near Central City a number of porphyry dikes occur. || The largest 

 one of these is located on Quartz Hill. Several of the others, Bobtail, 

 Gregory, and Mammoth Hills, show similar dikes near their highest 

 points. Essentially, the rock is a quartzose porphyry. Its paste is very 

 compact, light green to yellow, gray, and brown. On Gregory Hill very 

 tine crystals of orthoclase are contained in the porphyry. They occur 

 simple and as Carlsbad twins. Owing to the composition of the i)aste, 



* Eep. U. S. Geol. Surv., 1873, p. 325. § Eep U. S. Geol. Surv., 1873, p. 328. 



t Eep. U. S. Geol. Sarv., 1873, p. 338. || U. S. Eep. Geol. Surv., 1873, p. 281. 



X Eep. U. S. Geol. Surv., 1873, pp. 144, 686, and 688. 



14 G- 



