72 QUICKSILVKU DEPOSITS OF THH I'ACIFU; SL(jrE. 



mass must tend t(i a splierical form, provided tliat It is of uniform composi- 

 tion. 



If tlie l)odv is permeable to different depths in differe'.t directions or 

 if it offers more resistance to abrasion in one direction than it does in others, 

 the surfaces whieli offer the least resistance will e%idently be most rapidly 

 attacked. Hence, pebbles of sedimentary rocks, which do not in general 

 possess equal coherence in all directions, will not tend to a spherical form, 

 but to one more or less approaching a spheroid or even an ellipsoid. 



It ai)pears from the literature of geology that rounded masses resulting 

 from the decomposition of comparatively impermeable rocks have not 

 infrequently been mistaken for water-worn pebbles. Wlien one considers 

 that in both cases the approach to the spherical form is due to similar causes 

 this does not s»em so strange as it otherwise might. 



CRYSTALLINE METAMORPHIC ROCKS. 



Groups of metamorphic rocks. — Thc metamorpliosed Tocks of the Coast Ranges 

 may be divided into serpentine and a more or less crystalline series. The 

 latter, indeed, usually contain some serpentine; but serpentinization is evi- 

 dently in part a secondary process and will be discussed, together with the 

 massive serpentines, in a succeeding section. The division of the crystal- 

 line series which appears best to satisfy botli their microscopical charac- 

 ter and their field occurrence is as follows: (1) Partially metamorphosed 

 sandstones, in which, although a process of recrystallization has begun, the 

 clastic structure as seen inider the microscope is not obliterated, though 

 more or less obscured. These rocks will be referred to hereafter, for the 

 sake of brevity, as altered sandstones. ('2) Granular metamorphics, in which 

 thorougli metasomatic recrystallization of the sandstones has transformed 

 the mass into a granular, holocrystalline aggregate which, in its most com- 

 plex development, consists of augite, amphibole, feldspar, zoisite, and 

 quartz, witli accessor}' minerals. This class cannot be sharply separated 

 from the first or from the following, but it forms a natural group, one or 

 several of the constituents of which may be suppressed, forming different 

 varieties within the group, (o) Glaucophane schist of an origin similar to 

 that of the granular rocks, U'^uallv carrving mica, quartz, and other minerals. 



