116 QUICKSILVER DEPOSITS OF THE TACIFIC SLOPE. 



in the rocks with wliich it is geog-nostically associated. Basalt, it is true, is 

 common in some districts where serpentine abounds, but tliis basalt appears 

 to be Post-Tertiary, while the serpentinization took place early in the Cre- 

 taceous. At New Almaden also pebbles of an olivinitic gabbro have been 

 found, which probably come from the neighborhood of Mt. Bache. This 

 rock does not occur in place at New Almaden and is very fresh, the olivines 

 showing the merest traces of serpentine. Macroscopically it strongly re- 

 sembles the gabbros from Mt. Diablo and the Great Western, which contain 

 no olivine. It is not impossible that some of the serpentine of the Coast 

 Ranges may have been derived from an olivinitic rock like this, but if so 

 the quantity thus formed must be inconsiderable comi)ared with the rest, 

 since no trace of it has been detected elsewhere. 



The greater part of all the serpentine shows more or less perfectly 

 developed grate-structure. In polarized light the bars usually give higher 

 colors of interference than the interstitial matter, but this relation is some- 

 times reversed. With low powers a single bar often appears to extinguish 

 light simultaneously all over, but when more magnitied the extinction is 

 seen to be undulous and to correspond to the composite nature of the bars, 

 which are made up of foils or fibers in nearl}- parallel positions. The inter- 

 stitial matter often gives very faint and undulous colors of interference, but 

 has not been found isotropic. The more regular grate-structure passes 

 over by gradations into a less symmetrical disposition, and felted fibrous 

 masses are not uncommon in which irregular patches show slight differences 

 of tint both in polarized and in natural light. 



In the altered sandstones many substances are of course included, and 

 in some cases it might be questioned whether these rocks should be classed 

 as serpentines with very abundant inclusions or as sandstones carrying 

 much serpentine. So, too, a part of the granular metamorphics contain a 

 very large amount of serpentine. In the purer serpentines residual grains 

 are not abundant, but augite may sometimes be observed with cleavages 

 parallel to the grate-bars. These isolated augite grains show apjjropriate 

 oblique extinction and characteristic colors of polarization Chromic iron 

 is a common inclusion in the purer serpentines, but by no means invariably 

 present. It is of a deep, dull-brown color, transparent, monochn)itic, iso- 



