532 GEORGE DAVIS LOUDERBACK 



so abundant and well known in various parts of northern and central 

 California. They are therefore provisionally placed in a group by 

 themselves. Their relationship to the Dillard series was not definitely 

 determined. As typical pebbles were found in the Myrtle con- 

 glomerate, they are undoubtedly pre-Myrtle. 



The ultrabasic rocks.— Serpentines occur abundantly in all of the 

 major areas of the Dillard, sometimes in large masses or elongated 

 areas traceable for miles, frequently in small patches or dikes down 

 to exposures a few yards across, scattered irregularly through different 

 parts of the series. The small dikes do not appear to have any close 

 spacial relationship to any larger or more central masses, but occur 

 sporadically and independently at any point of the areas. Where 

 not badly disturbed, or on fresh fracture of less altered masses, these 

 rocks are dark — almost black, with a brown or green tinge. Most 

 of the material has undergone considerable movement, and is filled 

 with shear zones and slickensided surfaces, with bowlder-like residual 

 masses in greater or less abundance which show the real texture on 

 breaking. The serpentine has generally a more or less massive 

 structure to the eye and was largely derived from olivine. Fresh 

 fractures show, as a rule, a generous sprinkling of foliated crystals, 

 with a pearly to almost metallic luster, and with the appearance of 

 phenocrysts in the compact-looking ground. These are sometimes 

 distinctly bastite, and then presumably derivatives of enstatite or a 

 similar rhombic pyroxene; at other times the more or less altered 

 original pyroxenes — enstatite or diallage. These pyroxenes may 

 locally make up the entire mass and produce pyroxenites. Other 

 transitions occur to a feldspathic rock, and the gabbros described 

 above under (a) appear to be differentiation products of the magma 

 which gave rise to these serpentines. 



As the serpentines are very resistant to weathering, and give rise 

 to very little soil in situ, the greater masses are generally bare of 

 vegetation, and the larger areas, which frequently form hills or 

 ridges, give striking exposures easily traceable by the eye at a dis- 

 tance. The small dikes, however, frequently cause sliding in their 

 vicinity, and are more or less covered with debris from topographically 

 overlying formations, so that they produce very inconspicuous expo- 

 sures, and their presence and extent may become very difficult to 

 determine. 



