AMERICAN JOURNAL OF SCIENCE. 
[THIRD SERIES] 
and Potsdam Sandstones, and in certain Archean Quartzites, 
in ; 
[Published by authority of the Director of the United States Geological Survey.] 
In his address before the Geological Society of London, 
delivered Feb. 20th, 1880, Sorby describes sands whose grains 
are bounded externally by crystalline faces, but have on the 
gli is in “perfect optical and crystalline continuity ” with 
the interior grains, each broken fragment having been changed 
toa “definite crystal.” He shows, further, that such crystalline 
Sands occur in the sandstones of various ages “ from the Qilites 
down to the Old Red,” and that they are commonly little 
coherent, but that in some specimens ‘a number of grains may 
often be seen cohering more strongly than the rest; and these 
Show clearly that the cavities originally existing between the 
See have been more or less completely filled with quartz. 
oreover, on carefully examining the less coherent grains by 
Surface-illumination, we can see, not only the planes and angles 
_ due to unimpeded crystallization, but also more or less deep 
Mpressions due to the interference of contiguous grains, thus 
Proving conclusively that the deposition of crystalline quartz 
took place after the nuclei were deposited as a bed of normal 
4M. Jour. an Series, Vor, XXV, No. 150,—Junz, 1883. 
