Palache.] 



Lherzol ite- Serpen tine, 



163 



derivation from an eruptive rock in this area, and thus add one more 

 to the short list of such occurrences as yet described on this coast.* 



THE SERPENTINE. 



Outcrops. — The serpentine appears in rounded hills, which are 

 bare and strewn with rock fragments of local derivation, or are cov- 

 ered with a scant soil, probably largely of seolian origin, in which 

 similar fragments of the underlying rock are imbedded. These 

 fragments are mostly small and rounded by weathering. In places 

 the outcrops protrude above the soil to a height of five or six feet, 

 presenting irregular pitted surfaces fashioned by weathering into 

 most fantastic shapes. Where the surface is free from rocks there is 

 generally a poor growth of grass, plainly showing the thinness and 

 poor quality of the soil beneath, which, except in alluvial depres- 

 sions, is never more than a few inches in depth. 



Pctrographical Character. — The serpentine assumes at least two 

 well-marked facies, which for the purpose of description may be 

 designated the " slickensided facies" and the "massive facies." 



' The slickensided facies, which composes the greater part of 

 the serpentine mass in question, is chiefly characterized by the 

 abundant evidence of internal movement which it displays. This 

 evidence is of two kinds: 1. The presence of surfaces both plane 

 and curved, intersecting the rock mass in all directions, along which 

 relative motion of the rock particles has taken place. 2. The fault- 

 ing of veins of secondary minerals in the rock and the bending of 

 crystals imbedded in its mass. 



The presence of these ^surfaces of movement gives rise to the 

 appearance suggested by the name given to this facies of the ser- 

 pentine, each surface being in reality a slickensided surface. So 

 numerous are these surfaces that no fragment of the rock, however 

 small, can be obtained which is not bounded by them. They are 

 sometimes parallel or concentric, and so close as to give the rock a 

 leafy or scaly structure, which causes it to rapidly crumble away 

 when exposed to the atmosphere. The surfaces are smooth and at 



*M. E. Wadsworth, " Lithological Studies," Vol. XI, Part I. Mem. Mus. 

 Comp. Zoology, Harvard College, 1SS4, pp. 129-132, 15S. Turner, H. W., 

 " Geology of Mt. Diablo," Bull. Geol. Soc. Amer., Vol. 2, 1S91, pp. 388-391. 

 Merrill, G. P., footnote to above, p. 389. 



