402 R. D. Irving—St. Peters and Potsdam Sandstones. 
and. The very imperfect consolidation sometimes met with 
is, perhaps, not so very surprising, when we reflect on the very 
small coherence of many large quartz crystals which are yet in 
close juxtaposition. However, it does seem probable that this 
erystallization of quartz sometimes contributes very materially 
to the cohesion of the grains in hard and compact quartzites. 
In such examples as the Gannister of the South Yorkshire 
coal-field we can see, in a thin section, that the grains fit along- 
side one another in a very striking manner, and it is only by 
extreme care that good proof can be obtained of the actual 
deposition of quartz between them. owever, in the case of a 
highly consolidated sandstone from Trinidad, the proof of the 
deposition of quartz is as complete as possible; the outline of 
the original grains of sand is perfectly distinct, and the cavities 
between them are filled with clear quartz in crystalline contin- 
uity with the contiguous grains, so that the whole is a mass 0 
interfering crystals, each having a sand-grain as a nucleus. 
The rock has thus been converted into a hard quartzite, almost 
lized in situ. All my specimens of these quartz rocks are 
really highly quartzose mica-schists ; and, so far, I have failed 
in my endeavors to trace the connection between them an 
true sandstones, though possibly this could easily be done 12 
some districts which I have not examined.” 
These very highly interesting and important observations of 
Sorby have received surprisingly little attention, an had 
missed them altogether, so that it was not until after I had 
But more than this, 
to this deposition of interstitial quartz, I find certainly true. 
in certain Archean quartzites, and 
crystalline schists of the northwestern states, 1 select . 
quartzites as likely to yield the most readily, to this sort of 
