COMPOSITION OF ELEOLITE. 235 



Another of the prominent components is eleolite, which appears as 

 irregularly shaped masses or as large columnar crystals with a length of 

 as much as two inches and a breadth of halt' an inch. The irregular 

 masses are distributed uniformly throughout the rock, while the crystals 

 occur only in those portions in which the darker constituents are lacking 

 ( i. <>., in acid " Schlieren v ). In both cases the mineral possesses a gray 

 color and the characteristic oily luster of eleolite, while its cleavage 

 cracks are marked by interpositions of long dark needles of a black mica. 

 Dr. Clarke* reports the eleolite to contain — 



Si0 2 ALA CaO MgO K 2 Na,<) II,<> Total 



13.74 34.48 tr. tr. 4.55 16.62 0.86 100.25 



All the constituents above mentioned are usually imbedded in a fine 

 sugary aggregate of feldspar, of which there are several varieties, as will 

 he shown later. Occasionally this fine grained aggregate is in very large 

 quantity, when it appears as a groundmass surrounding the coarser 

 grains. More frequently it is in smaller or larger areas between the other 

 components, and in rare cases it is entirely absent. In this latter event 

 the rock is a coarsegrained mixture of large albite and eleolite grains 

 and plates of lepidomelane. Its structure is massive, while that of all 

 other varieties is schistose. In these schistose phases the plane of schis- 

 tosity, as shown by the lamellar arrangement of the mica plates, is par- 

 allel to the contact of the rock with a lepidomelane schist, that is prob- 

 ably nothing other than a very basic portion of the rock magma that has 

 been rendered schistose by pressure. In thin sections of all specimens 

 in which the schistosity is marked, the foliation is plainly seen to be due 

 to pressure; for not only are the feldspars marked by many series of 

 curved twinning lamellae, but the rock is also shattered, and in the cracks 

 separating its different portions a large quantity of new feldspar has been 



deposited. 



Microscopic Description. — The texture as revealed by the study of thin 

 sections is thoroughly granitic, in that nonef of the components possess 

 crystal outlines, though many of the eleolite grains and some of those of 

 the albite have quite well defined rectangular cross-sections. With the 

 exception of the rare zircon, the lepidomelane is the oldest constituent, 

 l.ut whether this is followed by eleolite or albite it is difncull to deter- 

 mine, since in mosl cases the eleolite and the larger grains of albite 

 are separated by areas of finer grained feldspars that are certainly 

 later in origin than either one of the two minerals mentioned. It is 



*Ibid., p. 262. 



; ih i -i,, i imeni applies only t" the main m iss of the rock, and is nol true » il h regard to il 

 or basic aggregations ("Schlieren"), where crystals of olcolite oi of lepidomelane are nol un 

 common. 



