Correspondence. 42J5^ 
On agitating a portion in water it was found that a considerable part 
was exceedingly fine and remained in suspension, while the remainder 
subsided as a granular powder of nearly uniform grain. The specific 
gravity of the latter as determined by the specific gravity bottle was 
2.433. 
Under the microscope the dust is seen to be composed chiefly of frag- 
ments of glass and crystals and crystal fragments of hornblende and 
feldspar. 
The glass is much the most abundant material. The fragments, 
which are all sharply angular, vary in size from less than .01 mm. up- 
to .28 mm. in diameter, the average diameter being about .05 mm. 
The glass is for the most part compact, clear and colorless, but occa- 
sional pumiceous fragments were observed and others stained brown or 
yellow by iron oxide. 
The feldspar is next in order of abundance and occurs in prismatic 
crystals and in angular crystal fragments. The largest crystals ob- 
served measured .14 mm. to .308 mm. in length and .084 mm. to 
.126 mm. in breadth. Some of the fragments were recognized as plag- 
ioclase by distinct polysynthetic twinning, but the majority appeared to 
be simple crystals and are perhaps orthoclase. Tiny inclusions of mag- 
netite were observed in some of the feldspar crystals. 
The hornblende is slightly less abundant than the feldspar, but the 
crystals are of larger average size. The largest crystal observed meas- 
ured .3 mm. in length and .1 mm. in breadth; the average was about 
half that size. The hornblende is readily recognized by its prismatic 
habit and cleavage, green color, strong pleochroism, and small extinc- 
tion angle. 
In the powder that was washed with water several shining cleavage 
fragments were observed. Under the microscope, one of these gave 
rather uncertain reactions for pyroxene. It is present in very small 
quantity. 
Rather numerous particles of magnetite are present, the largest of 
which measured about .050 mm. in diameter. Many of these grains had 
fragments of glass or feldspar attached to them. 
The mineralogical composition and specific gravity of this volcanic 
dust would point to its derivation from a somewhat acid andesitic 
magma. Charles Palache. 
Oeological Laboratory, University of California, March 20th, 1893. 
Beltrami Island of Lake Agassiz. After returning last month 
for a season of work on the Minnesota Geological Survey, one of my 
earliest duties was to examine the profiles of the three railroad lines, 
with their branches, which have recently been built (and are now being^ 
extended) from Duluth northward across the rich iron and lumber dis- 
trict of northern Minnesota. One of these, the Duluth & Winnipeg 
railroad, passing northwest by the east end of Red lake and the south- 
west side of the Lake of the Woods, shows that the former of these lakes 
lies about 40 feet and the latter somewhat more than 150 feet below the 
