390 GEOLOdY OF THE EUREKA DISTlllCT. 



by Miller fur thermoplillite, u foliated mineral Jiaviiig' tlie composition of serpentine, 

 concerning wbich Prof. Dana remarks that it seems i^robable both that this is "truly 

 crystallized serpentine" and that "the crystallization of this species is actually mica- 

 ceous, like that of chlorite and talc."' The red, completely altered, macroscopic olivine 

 is seen in the hand s]5ecimen to have a glistening, mica-like cleavage surface. There 

 remained in the portion subjected to acid well developed, nearly oiiaque octahedrons, 

 most likely iiicotite. The crystallization of the olivine appears to have preceded that 

 of the other minerals by only a short period, stopping just as they began, for no 

 inclusions of them are found in the olivine except around its border, where augite and 

 more rarely feldspar are seen penetrating its surface, causing it to have an uneven and 

 broken outline. 



A less important, though ever present, component is magnetite, which is very 

 abundant in well crystallized octahedrons of from 0"03 to O-Ol """ and less. It is very 

 uniformly scattered through the rock, and in some cases, thin sections 285, 282, 283, 

 is closely associated with augite, attaching itself to the surface of the crystals and 

 being included in them, but it is seldom seen penetrating the substance of the feldspar 

 crystals. It is everywhere fresh and there is no evidence under the microscope of the 

 presence of titaniferous iron. As accessory minerals the only one to be mentioned is 

 quartz, which is found in one or more macroscopic grains in nearly every hand 

 specimen, but Avhich is rarely met with in the thin sections. Tliere is a fragment in 

 thin section 2G1, which is 1-4 ■"™ long, with an angular outline and conchoidal fracture. 

 It is cut exactly at right angles to the optic axis, yielding a slightly distorted inter- 

 ference cross and proving to be a single individual grain. It contains a crystal of 

 zircon, several hair-like trichites, and a few irregularly shaped fluid inclusions with 

 very broad dark borders, carrying extremely active gas bubbles, which disappear at 

 a very slight elevation of temperature, thus indicating the fluid to be liquid carbon 

 dioxide. There is besides these inclusions a diliexahedral cavity filled with glass 

 and a comparatively large crystalline grain which crowds the bubble of gas out of its 

 usual spherical shajie. The character of its inclusions, the unity of the whole quartz 

 as a single individual, together with the surrounding shell of augite crystals, leaves 

 no doubt of the primary nature of this quartz, which corresponds in a measure to 

 that of quartz-porphyry. There are besides in some thin sections amygdules of a 

 green or red, radially fibrous, delessite-Iike mineral, and in others microscopic aggre- 

 gates of one of the zeolites, whose action on polarized light and apparent polysyn- 

 thethic twinning suggest the characteristics of chabazite as given by MM. Fouqu^ 

 and Michel-Levy. 



Glass is more or less abundant in all the basalts from the localities mentioned. It 

 is for the most part colorless in thin section. In a few instances it is brown, and in 

 the darker varieties it swarms with seemingly black globulites, which, however, are 



'J. D. Dana. A System of Mineralogj, 5th ed., p. 465. 



