The Koochiching Granite. — Winchell. 293 
THE KOOCHICHING GRANITE. 
By Alexander N. Winchell, Koochichiug, Minn. 
On the northern boundary of Minnesota, two miles west of 
Rainy lake, is an important waterfall. The Rainy river here 
plunges downward about twenty-five feet over a rock which 
the full force of the torrent but slowly wears away. This 
bed rock of the falls is considered to be of eruptive origin, 
and seems to belong to the Laurentian age.* The river at 
this point is near the southern edge of the eruption, which 
extends away to the northeastward for about fifteen miles, 
while its liniits on the north and west are not definitely 
known. 
Macroscopically the rock,f in the region selected, is a crys- 
talline, medium-grained mixture of : 
1. A plagioclase feldspar, frequently exhibiting twinning. 
2. Another feldspar, exhibiting no twinning, but occasion- 
al cleavage. 
3. Mica, which is quite abundant and easily distinguished. 
4. Quartz, which is less abundant than is common in 
granites. 
5. A green mineral resembling hornblende. 
The general color of the rock is dark gray, weathering to 
nearly black. Weathered surfaces further show that the 
darker materials (mica and the green mineral) are the first to 
disintegrate. 
A separation of the rock constituents by the specific grav- 
ity method resulted as follows: 
Mica 12.00 per cent. 
Above 3 9.29 " 
Between 3. and 2.661 34.66 
Between 2.661 and 2.642 25.02 " 
Between 2.642 and 2.58 7.60 " 
Below 2.58 11.43 " 
Total 100.00 " 
It is almost needless to remark that the accuracy of the de- 
termination is much more apparent than real; indeed, 1.95% 
of the powder was lost in the operations. 
*A. C. Lawson : Geol. Surv. of Can. Ann. Rep., 1887. Pt. CC, P. 
126 F. 
tThis description is based on specimens 1030G and 1030aG of the se- 
ries of the Geol. and Nat. Hist, Survey of Minn. 
