SUMMARY. 455 



composition points to the action of organic acids, especially the hnnius 

 acids, and that the class of concretions of which this is a type must have 

 contained nuclei of organic matter which have decomposed and disap- 

 peared. 



Rounded nodules resulting from the action of (lecomi)osition processes 

 on angular masses are discussed, and it is shown that the rapidity of attack 

 must be in an inverse ratio to the radius of curvature of the mass. This 

 explains the fact that such nodules tend to a spherical form. The rounding 

 of })ebljles and of sand grains is shown to depend on the same mathemat- 

 ical law. 



Sharply defined limits cannot be drawn between the various early Cre- 

 taceous metamorphosed rocks of the Coast Ranges ; they pass over into one 

 another by degrees. For purposes of description, liowever, it is desirable 

 to consider certain types as distinct. The divisions which appear to satisfy 

 best both their field occurrence and their microscopical character are as 

 follows: rartiall// nidamoyphosed sandstones, in which, although a process of 

 recrystallization has begun, the clastic structure as seen under the micro- 

 scope is not obliterated, but is often more or less obscured. This class will 

 be referred to hereafter for the sake of brevity as altered sandstones, (han- 

 idar mctamorphics, in which metasomatic recrystallization of sandstones has 

 transformed the mass into a holocrystalline aggregate, form another group. 

 The tliird class embraces the (jlaucophane schists, derived from certain shales, 

 nuich as the granular metamorphics are produced from sandstone. The 

 pLtliaiiifes are a series of more or less calcareous, schistose rocks wliichhave 

 been suljjected to a process of silirification, resulting in chert-like masses, 

 which retain schistoid structure and are intersected by innumerable (piartz 

 \eiiis. They usually carry more or less zoisite. Finally the serjyoifines, 

 which have resulted in part from the direct action of solutions on sandstones 

 and in part from alteration of the granular metamorphics. 



A considerable number of minerals have been generated in these rocks 

 by metasomatic processes and weathering. These are biotite, muscovite, 

 augite, hornblende, glancophane, labradorite, andesine (probably), oligo- 

 dase, albite, orthoclase, quartz, zoisite, rutile, ilmenite, titanite, apatite, 

 garnet, nacrite, chlorite, epidote, serpentine, and chromite. The most inter- 



