LINCOLN PORPHYRY. 329 



than ill the type rock, but in otliers they caiiDot be well (listinguished from the tri- 

 clinic feldspar. Plagioclase is always very abnudant in white iudividuals, seemingly 

 less fresh than the orthoclase, although a stnatiou can often be seen on the bas;il 

 cleavage surfaces. Biotite occurs in small hexagonal leaves, which are sparingly but 

 uniformly scattered through the whole. They are seldom fresh and usually appear to 

 be changed into a green chloritic mineral. The quartz appears as a prominent luacrc- 

 scopical constituent, showing, as a rule, a development of pyramidal planes, to which 

 the prism is occasionally added.' The groundmass is dense and homogeneous in appear- 

 ance, usually grayish in color in fresh rocks, and very distinct. Only occasionally does 

 it become subordinate. Ore particles are plainly distinguishable in it. 



Specimens of the rock obtained from exposed surfaces of high mountains are 

 usually bleached and light-gray in color, sliglitly stained by hydrous oxide of iron, 

 while in tunnels and mine workings the rock is generally greenish through the chlo- 

 ritic decomposition products of the biotite. 



Microscopical — The mici'oscopical examination reveals the following as original 

 constituents, named in order of formation, viz: Allanite, zircon, magnetite, titanite, 

 apatite, biotite, plagioclase, orthoclase, and quartz. All the minerals named occur in 

 more or less perfect crystal form and are imbedded in a giauular groundmass, consist- 

 ing of plagioclase, orthoclase, and quartz. The amount of plagioclase iu the ground- 

 mass is doubtless small, for it is so abundant in the form of imbedded crystals that 

 but little substance could have remained for the second generation. The size of the 

 grains in the groundmass is so small that one cannot well distinguish between quartz 

 and orthoclase, but the holocrystalline nature is evident. ITo microfelsitic or glassy 

 matter has been found in any rock of this type. 



Of the accessory constituents the most noteworthy is allanite, which appears 

 very sparingly but constantly in this and other rocks of the Mosquito Range and 

 adjoining regions. It is apparently the first mineral formed, or is perhaps contempo- 

 raneous with zircon, these two minerals penetrating even magnetite and apatite. 

 During the first study of these rocks the nature of this mineral was not determined, 

 but, through the subsequent detailed investigation of a similar porphyry of the Ten- 

 Jlile mining district, enough was isolated by means of the Thoulet solution to allow of 

 chemical analysis. The analysis, made by W. F. Hillebrand, was not completed, owing 

 to accident, but it established the presence of Ce and La with the absence of Di, while 

 Fe^Os and SiOz were the remaining constituents of note. At about the same time Mr. 

 Joseph P. Iddings, of the U. S. Geological Survey, determined the same mineral 

 crystallographically in various rocks of the Great Basin in ISTevada. As a rule 

 the allanite is seldom macroscopically visible in the rocks of the Jlosquito Range, 

 while it is quite noticeable in those of the Ten-Mile region. It appears in small 

 piisnis of maximum length of about 5'"", has a brilliant dark resinous luster, and 

 when decomposed stains the surrounding zone in reddish-brown shades. The chance 

 sections show a transparent, yellowish brown mineral, with no distinct cleavage. 

 The faces developed seem probably refei'able to a P, co Pec , OP, and + Pab . It 

 is often twinned, possibly parallel coPoo , as by epidote, and in several sections which 



' On the ridge cast of Hoosier pass the outcrop of a porphyry sheet is marked by quartz crystals 

 which have weathered out of the underlying rock and which ^how both pyramid and prism. 



