ARCOSE. 61 



shown to be composed of granitic detritii.s, or, in other words, to bo arcose. 

 In many cases, indeed, it appears from structural considerations improb- 

 able that the sands were immediately derived from granite and altogether 

 probable that they were formed by the disintegration of earlier sand- 

 stones. The microscope shows, however, that some of these rocks consist 

 of grains of such angularity and sharpness as to lead inevitably to the con- 

 clusion that they were directly derived from granites — indeed, from granites 

 at no great distance from the point of deposition. As a rule the grains are 

 worn and rounded like ordinary beach sand, and in such cases tlie micro- 

 scope fails to show whether the material was immediately or indirectly de- 

 I'ived from the original granite. But the arcose character is persistent in 

 all these rocks, and this points to short transportation; for the admirable ex- 

 periments and observations of Mr. Daubree prove that the feld-spathic con- 

 stituents of granite are rapidly triturated and decomposed in running water. 

 I cannot recall any description in geological literature of a mass of arcose 

 so immense as that exposed in the Coast Ranges. 



All the characteristic components of granite reappear in the sandstones, 

 often in proportions differing but little from those which prevail in the 

 parent rock, and it is very rarely the case that the sandstones contain any 

 clastic fragments or allothigenetic minerals not identified in the granites 

 still exposed in the Coast Ranges. Chemical analysis is not calculated to 

 exhibit the origin of the sandstones, for in the course of disintegration 

 and transportation a certain amount of material must have been reduced 

 to impalpable powder, decomposing agencies cannot have been altogether 

 absent, and a certain amount of mechanical concentration must have taken 

 place, although this last influence was reduced to a minimum by the close 

 approach of orthoclase to the density of quartz. It is manifest that the 

 chemical indifference and superior hardness of the quartz establish a ten- 

 dency to greater acidity in the sandstones than in the granites. 



Microscopical character. — Tlic quartz of tlic frcsli aud of the slightly decom- 

 posed sandstones is exactly similar to that of the granite and commonly 

 contains abundant fluid inclusions, those of small size and regular form 

 showing active bubbles. The feldspars ai-e often present in about the same 

 quantities as the quartz. The predominant species is orthoclase, with char- 



