102 W. H. HOBBS — METEORITE FROM A LOOM A, WISCONSIN 



common direction, the radius of curvature diminishing as the circum- 

 ference of the meteorite is approached. They represent, therefore, the 

 arms of an Airy's spiral. This variation from a straight line is approxi- 

 mately 1 in 30 or 1 in 50, and is greatest where the slope from the 

 central boss is the steepest. In only one instance was any variation 

 from the common direction of curvature to the left (la^vo-rotation) 

 observable, and this was in the bottom of the larger of the two marginal 

 pits, where for a short distance the curvature is reversed only to re- 

 sume its regularity near the edge of the disc (see upper part of plate 5). 



Surface markings due to crystalline structure. — Beneath the prominent 

 drift scorings the lens seldom fails to reveal the crystalline structure of 

 the metal in regular cross-linings of lesser prominence. These show to 

 the best advantage in the bottom of the deepest marginal pit on the side 

 nearest the center of the mass — in the '' lee of the wall " of that side. 



Fracture lines. — Perhaps in some way connected with the crystalline 

 structure is the series of parallel cracks which course over the front of 

 the meteorite in a direction about parallel to its greatest cross-diam- 

 eter. These joint-like cracks are observable by the unaided eye, and 

 can doubtless be made out in plate 3. Their direction is more clearly 

 indicated in figure 1. In places they are crowded together, separated 

 by rather uniform space intervals of 1 to 2 millimeters, and at times 

 they are lined with oxide, especially on the more oxidized side of the 

 front. 



THE CONCAVE SURFACE 



^^ Thumb marks.^'' — The generally concave surface of the meteorite, as 

 already explained, is in sharp contrast with the convex surface or front. 

 In general aspect it does not differ from the surface of the greater num- 

 ber of meteoric irons, and may be described as undulating, due to the 

 presence of large and very shallow pits (" thumb marks ") which coalesce 

 with one another. 



Oxide scale. — Over all this surface of the meteorite is a coating of oxide 

 of iron which varies in thickness from less than i to about 1 millimeter 

 in thickness. Thickest on the side of the most protected hollow of the 

 surface, it has scaled off locally and left a series of irregular depressions 

 on the larger pittings of a second order of magnitude (see plate 4). As 

 this edge is the one which corresponds to greater oxidation upon the 

 front, it is probable that some small portion of the oxidation occurred 

 subsequent to the fall, due to the unfavorable conditions for preservation 

 as regards air and moisture. 



On this surface of the meteorite there are no distinct markings observ- 



