Enlarged Sandstone Groins. — Calvin. 227 
crumbled between the fingers. If the detached grains are 
placed on a black surface and examined with a good magni- 
fier their crytalline character becomes apparent, and the 
angles and faces may be made out fairly well. Examined 
with a compound microscope having a low power objective, 
the crystalline angles and facets can be more satisfactorily 
determined. Mounted in balsam and viewed by transmitted 
light, the boundary between the original sand grains and the 
chemically deposited envelope becomes in all cases apparent, 
on account of the thin layer of ferric oxide with which the 
grains were originally coated. With polarized light the opti- 
cal continuity of nucleus and envelope is readily demonstrated. 
The localities in Allamakee county, Iowa, when- this crys- 
tallized sandstone is exposed are very numerous. It may be 
seen in place by the side of the road in the S. W. j of Sec 2(5. 
T. 98, R. 5 W., about three miles east of Waukoii, and blocks 
of it are strewn for a mile or two down the valley that leads 
toward Village creek. In the valley of Waterloo creek the 
same sandstone appears in section 25 of Waterloo township; 
while in the other direction, it crops out in the washes and dry 
runs above Waterville, in the valley of Paint creek. The fact 
is, it is found at hundreds of local exposures, — wherever, in- 
deed, the undulating surface of the region intersects its hori- 
zon, — in a broad belt several miles in width, extending from 
the northwest corner of Allamakee county to the mouth of 
the Yellow river. 
American geology is indebted to Irving and Van Hise for 
practically all that has been written on the secondary enlarge- 
ment of quartz grains in sandstones. It is not intended here 
to add anything to the descriptions and explanations of the 
phenomena of enlargements given by these authors in the 
Fifth Annual Report of the U. S. Geological Survey, in Bulle- 
tin No. S of the same survey, and in papers published in the 
American Journal of Science, The only purpose of this paper 
is to call attention to a geological horizon not enumerated in 
their list, and to a number of localities where one may collect 
specimens of sandstone exhibiting in remarkable perfection 
the interesting peculiarity of having facetted grains resulting 
from the deposit of an envelope of silica in optical continuity 
with the waterworn nucleus. 
