METEORITES OF NORTH AMERICA. 27 



among the neighbors, but as yet with no positive results. As will be shown below, there was reason to think that 



fragments might exist in the vicinity and the search was continued with the aid of dial compass and dip needle, but 



without success. 



Size and Shape. 



Instead of the usual irregular form or the paraboloid shape of some oriented meteorites (Hraschina, Allegan, Long 

 Island), the Algoma iron is almost unique in having a discoid or shieldlike form. In the surface of greatest extension 

 the outline is roughly elliptical, with major and minor axes 25 and 16.5 cm. From a thickness of about 2.5 cm. near 

 the geometric center the disk varies irregularly, generally to smaller values and locally even to a knife edge at and near 

 the circumference. The convex surface in the plane of the minor axis of its outline and its normal has a radius of 

 curvature of about 21 cm., and the concave surface a considerably larger value, about 32 cm. The two broad sur- 

 faces are spoken of as the convex and concave surfaces, respectively, because the former invariably recedes near its 

 margin (though concave at one place and in one plane near its center). The other surface is more nearly concave than 

 convex, and in one plane (that of the minor axis and the normal) is distinctly concave. 



Weight and Specific Gravity. 



When brought to the university the Algoma iron weighed a little less than 9 pounds, or somewhat more than 4 

 kilograms (weighed with a spring balance). A small slice, in the widest place less than 3 cm. in width, was sawed 

 from one end, and due to a misunderstanding of instructions a saw cut was made, running partly through the meteorite 

 at a greater distance from the end. After suffering these losses the main meteorite mass now weighs 3,710 grams. 



A block weighing a little over 39 grams, polished on two sides, after boiling in water for half an hour and cooling 



to room temperature, was weighed in the water, and then, after drying, in air by the suspension method. The result 



obtained for ihg specific gravity was 7.75. 



Surfaces. 



the convex surface. 



Larger irregularities. — The convex surface, for reasons which will appear, designated the front of the meteorite 

 (Brustseite) , merits a careful consideration. Marks which this surface owes to its maltreatment subsequent to its dis- 

 covery in 1887 are the dents from pounding with a sharp rock edge, the grooves from attack with a cold chisel and 

 hammer, and the abraded surface and scratches which are the work of a file. In addition to these disfigurements, 

 there are * * * relatively deep pits of markedly irregular outline which doubtless owe their origin to the fusion 

 and removal of a mineral (schreibersite) more fusible than the nickel-iron itself. The manner in which such pits are 

 formed is well illustrated by the small pitting cut through by the saw, at the bottom of which may be seen the schrei- 

 bersite crystals separated by walls of swathing kamacite. One pit corresponds in position with a similar pit upon the 

 back of the meteorite and doubtless was once continuous through the disk, as it may now be followed nearly through and 

 appears to be choked for a short distance by the oxide scale which lines its walls. 



The low fusibility of schreibersite is well known and was well brought out during the polishing of the section, 

 the smaller crystals of schreibersite at once revealing their position by their fusion, due to the moderate frictional 

 heat of grinding. 



Larger shallow pittings, which more resemble the conventional "thumb marks," have doubtless been produced 

 in a similar manner by fusion of schreibersite, combined with fusion and abrasion of the outer walls, since their sides 

 toward the meteorite center possess the same steep, irregular slopes as the others, with some accumulation of oxide 

 scale. Being located near the circumference of the disk, they lie within the zone of maximum erosion from the action 

 of the compressed air, and the thin walls which presumably once separated them from the present circumference of the 

 meteorite would be hardly able to withstand the erosive action. 



The marginal area of the meteorite front, on which the "thumb mark" pittings are found (for convenience called 

 the straighter margin) is in rather sharp contrast with the opposite front margin (the lobate margin). From the 

 nearly plane central boss of elliptical shape (axes, 9 and 7 cm.) the surface of the disk slopes away sharply on the side 

 of the larger pittings and is there also deeply furrowed in directions nearly radial. The lobate side recedes in more 

 gradual curves, is marked by small and irregularly distributed pittings, and is covered with a thin film of oxide. 

 Through this coating of oxide the nearly radial furrows and ridges which characterize the opposite margin can be fol- 

 lowed without difficulty, though they are much less distinct, and the effect produced is altogether like that which 

 would be expected if this side had lain in a moist soil while the other had received greater protection. The 

 margin not covered by the oxide (that of the shallow pittings) shows a steely, metallic luster. Near the ends of the 

 front the surface resembles that of the margin where the shallow pittings are found. The end opposite the saw sec- 

 tion, however, projects to the front from the margin of the central boss before its surface recedes in the regular curves 

 characteristic of other parts of the front. This suggests that the meteorite may have been bent in about its geometric 

 center by a force acting normal to its surface. 



The central boss of the front shows, even under the lens, little trace of the radial furrows, and then only in circum- 

 ferential portions. On the straighter side, where the surface slopes away rapidly from this boss, the furrows begin 

 with great distinctness at the line where the flat boss gives place to the backward slope. 



Drift ridges and furrows. — The radial markings could perhaps better be described as ridges than as furrows; they 

 are in reality the material left in sharp, knife-edge lines between very shallow furrows having nearly flat bottoms. The 

 ridges have a basal thickness of a fifth to a tenth of a millimeter, and where best developed the intervening furrows 

 widen from about a millimeter at the margin of the central boss to two millimeters at the present circumference of the 



