AMERICAN METEORITES—HENDERSON 
was hot for about 2 feet from the place 
of impact. This stone fell upon dry 
grass, and grass was welded into the 
fractures of the mass by the impact 
but was not charred. Hence the 
meteorite itself was not very hot when 
it fell. The Colby, Wis., stone was 
recovered a few minutes after falling 
and when dug up and exposed to the 
air was so cold that frost immediately 
formed on the surface. Another such 
example of a cold meteorite is the 
Tilden, Ill., fall, parts of which were 
recovered at several different places 
along a path of about 3 miles, and 
each specimen was reported by dif- 
ferent finders as being cold when first 
touched. These various meteoritic 
falls all had fusion crusts over the 
outer surface, but the main mass was 
cold inside as there was not time enough 
during the flight for the interior of the 
mass to become even warm. 
The crust on freshly fallen meteorites 
may have delicate lines formed by the 
air dragging the softened material away 
from its foreward face (pl. 1, a). ‘These 
lines, or flight markings, reveal the ori- 
entation of the mass during the last mo- 
ments of its flight. Usually the crust 
is thin on the front face and shows 
some accumulation on the side and 
rear faces. No part of the crust is ever 
very thick, and since there are usually 
some silicate minerals containing iron, 
the crust on freshly fallen stones is 
generally black. Some of the rarer 
types of stony meteorites are made up 
of minerals so free of iron that the 
fused crust is cream-colored to gray. 
Freshly fallen iron meteorites are cov- 
ered with a black crust which has a 
submetallic luster. 
If the specimen is an old fall the 
crust probably has been altered to a 
brownish color on both the stony and 
iron meteorites. Prolonged weather- 
ing may further modify the crust until 
most of the flight markings are obliter- 
ated. In the case of stony meteorites 
prolonged weathering penetrates the 
mass and the entire stone becomes 
rather evenly discolored with brown 
hydrous iron oxide. Iron meteorites, 
817369—49——_20 
263 
being less porous, weather more gradu- 
ally than stones. Although weather- 
ing may be so extensive that the sur- 
face retains none of the original flight 
features, the interior of the iron may be 
bright, fresh metal (pl. 3, a). Iron 
meteorites have been found in which 
the entire mass has been altered to a 
brown oxide, but these are unusual 
examples. 
Another rather characteristic feature 
of a meteorite is the presence of shallow 
depressions on the surface, called 
‘thumb marks” because of the re- 
semblance to the imprint of the thumb 
on a plastic medium (pl. 2). These 
originate during the fiight of a meteor 
through the atmosphere and are 
caused by the scouring action of the 
air; since they are rather deep, they 
are not readily lost by weathering. 
These “thumb marks” are very dif- 
ferent in appearance from the rounded 
holes found in slags and furnace prod- 
ucts so frequently mistaken for meteor- 
ites. In fact, meteorites will not, on 
first inspection, appear to have been 
affected by heating. 
Meteorites are heavy, weighing more 
than the average terrestrial rock of 
equivalent size. Hence any unusually 
heavy specimen showing ‘thumb 
marks” and what appears to be a fu- 
sion crust over the surface should be 
investigated. The finder of such a 
specimen should spend several minutes 
looking around to become familiar 
with the variety of terrestrial rocks 
existing in that area. If by chance a 
meteorite has been discovered, it will 
become evident at once that the speci- 
men is noticeably different from the 
rocks in the vicinity. If there are nu- 
merous boulder or rock fragments scat- 
tered about, a genuine meteorite will 
differ in weight, shape, and crust from 
the average rock. Ifthe area contains 
very few rocks, then any single unusu- 
ally heavy specimen whose occurrence 
cannot be readily explained as being 
natural to the place should be looked 
upon as a possible meteorite. 
Many meteorites are made entirely 
of metal and hence are much heavier 
