SACRAMENTO PORPHYRITE. 341 



scopieally visible, proved to carry 65.73 per cent. SiO 2 . The range in silica is thus 

 more than 10 per cent., and it is chiefly affected by the amount of hornblende present 

 in the rock. 



SACRAMENTO PORPHYRITE. 



This rock, which was at first classed as a quartz-porphyry and is so represented 

 on the map, occurs in a large mass at the head of Big Sacramento gulch, whence intru- 

 sive bodies extend to the north and to the southeast. 



Description In structure this rock resembles those modifications of the Lincoln 

 Porphyry in which the formation of the large orthoclase crystals has been hindered. 

 It shows many white plagioclase crystals of the usual stout habit and a number of 

 smaller, less distinct individuals which are less fresh, most of them being ortho- 

 clase. Both biotite and hornblende are present in distinct individuals, and quartz 

 occurs in round grains, which are not very plentiful. The groundmass containing 

 these elements is subordinate but yet distinct. It contains pyrito and magnetite, and 

 a sufficient number of small biotite leaves to be dark-gray in color, when fresh. Chlo- 

 ritic decomposition products render it darker in most specimens collected. Epidote, 

 which is often prominent in more or less altered specimens, will be spoken of below. 



With the microscope it is found that zircon, allanite, .apatite, and titanic iron, 

 the last recognized by cleavage and alteration products, are, further components of 

 the rock. Allanite is not so plentiful as in some other types described, but it is ob- 

 served in one or two slides and is probably a regular but sparsely distributed con- 

 stituent. 



The microscope shows that plagioclase is developed almost exclusively in the 

 form of porphyritic crystals and that but few of these are certainly orthoclase, 

 although the latter mineral forms with quartz nearly the entire groundraass, the dark 

 silicates seldom appearing as constituent particles in this later product of consolida- 

 tion. Orthoclase is usually more decomposed than plagioclase, being cloudy after the 

 manner commonly seen in much older rocks. The majority of the plagioclase crystals 

 arc clear in the center, but show incipient decomposition in the. outer zones. The 

 laminae composing them are either broad or very narrow and the maximum angle of 

 extinction in the macrodiagonal zone is 20, indicating that varieties more basic than 

 oligoclase are rare if present at all. 



Hornblende and biotite have a thoroughly normal appearance, and are only 

 interesting through their decomposition products, to be considered below. Inclusions 

 of the early accessory constituents are a matter of course in all the large crystals, but 

 they are never abundant and are never accompanied by glass, so far as observed. 

 Fluid inclusions occur sparingly in quartz and feldspar. 



Decomposition products Noteworthy facts concerning the decomposition of rock- 

 building minerals may be observed in the Sacramento Porphyrite. The specimens 

 collected are divisible into two classes, the one showing a bleached rock, the other 

 containing macroscopically developed epidote. In the latter rocks [83-85] the micro- 

 scope shows a more or less marked tendency to the formation of epidote from both 

 feldspars, as well as from biotite and hornblende. In the last two minerals a dark, 

 strongly pleochroic chlorite is the forerunner of the epidote, while in the feldspar no 

 intermediate stage of any kind can be detected. Muscovite and calcitc, the common 

 products of alteration in feldspars, are here but slightly developed. 



