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PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



many small interfering crystals. Occasionally some of the sur- 

 rounding clear quartz is in crystalline continuity with these grains ; 

 but usually it has crystallized independently, probably because the 

 grains were originally completely surrounded by deposit, and not 

 by the empty spaces which existed in the case of the consolidated 

 sandstones with deposited quartz, already described. As an illus- 

 tration of one of these grains, I give fig. 9, magnified 30 linear. 

 In other cases the quartz of the grain is clearer, and the contrast 

 less, and in others so slight that it is impossible to be quite sure 

 whether they were originally grains of sand, or only detached parts 

 of the quartz which has crystallized in situ. It is, however, almost 

 certain that many such grains must be present, since quartz in 

 general is much more transparent than those grains which are 

 well marked in the manner described. The great difficulty of re- 

 cognizing clear grains must be borne in mind, since the apparent 

 absence of sand from some specimens of schist may be due to this 

 cause. 



Fig. 9. — Grain of Quartz-sand in tJie Mica-scMst of ArroquTiar. 



Along with the grains of quartz just described occur some of 

 what appear to have been felspar, much decomposed before the 

 deposit was altered into schist; and there are also some grains 

 very much as if they had been derived from a felsite. In some 

 respects these various felspathic grains are more distinct than those 

 of quartz, because they are granular and quite unlike the sur- 

 rounding rock. Possibly this is why they are sometimes seen in 

 specimens of schist in which no grains of quartz can be recognized ; 

 but still the evidence is not so rehable as in the case of milk-white 

 quartz, since one cannot but feel that it is always possible, even 

 when it is not probable, that they are portions of altered felspar 

 which crystallized in situ when the rock was metamorphosed. In 

 any case, taking all the facts into consideration, the proof of the 

 original presence of true grains of sand in some thoroughly crystal- 

 line and typical mica-schists is as perfect as in the case of slates. 

 Leaving doubtful cases out of consideration, about i of all my 

 sections of schist afford good evidence of the presence of such 

 grains ; and I find that also ^ of my sections of slates show similar 

 grains ; so that their absence from |- of my specimens of schists is 

 no valid objection to their having been originally of as true detrital 



