PSEUDODIABASE. 97 



one portion to the other, nor does the green nihieral exhibit greater iibra- 

 tion or any other structural pecuharity. In some cases the Hue of clemar- 

 kation is curvihnear, but nowhere does the green coh^r follow cleavages or 

 cracks or penetrate the brown by sharp indentation, as products of altera- 

 tion usually do. There seems nothing to indicate that the two varieties have 

 not formed simultaneously, and even on this supposition the sharpness of de- 

 markatlon and the character of the limits are difficult to understand. There 

 is a little ordinary chlorite in the slide, produced Ijy decomposition of horn- 

 blende. 



The augite possesses no peculiarity excepting its relations to tlie horn- 

 blende. There is some ordinary uralite, but tliere are also augite masses 

 partly surrounded by brown hornblende in sucli a way as to suggest a brown 

 uralite. The relation of the brown hornblende to tlie augite, however, i.; 

 similar to that which the green hornblende bears to the brown, and I cannot 

 satisfy myself tliat it is really epigenetic. 



The feldspar is for the most part clear and fresh, and a large portion 

 of it shows polysAuthetic structure. The extinctions observed indicate the 

 presence of oligoclase. To test the character of the feldspar, a separation 

 was made by the Thoulet method. At a density of 2.S5 a large precipi- 

 tate of bisilicates and zoisite fell, carrying a portion of the feldspars with it. 

 On reducing the specific gravity of the solution gradually, 5 per cent, fell 

 at 2.65, which appeared under the microscope to be pure feldspar. Between 

 2.6.5 and 2.58 there was no precipitate. From 2.58 to 2.56, 7 per cent, of 

 pure feldspar fell. This was found to contain 11 jjer cent, of soda, and 

 the rock, therefore, contains both oligoclase and albite. The zoisite occurs 

 in part in prisms with characteristic properties, but mainly as granular 

 masses, which are nearly colorless and monochroitic, but otherwise not 

 unlike epidote in appearance. Exactly such zoisite was isolated from a 

 Mt. Diablo specimen and chemically tested. Tlie greater part of the zoisite 

 is embedded in the clear feldspar, but it also fills interstices between feld- 

 spars, so that the formation of the two minerals must have gone on simul- 

 taneously. 



HON. xm 7 



