THE CONCAVE SURFACE 



103 



able which can by any probability be ascribed to erosive agencies within 

 the aerosphere. ' 



Infolding of edges. — Of some interest is the apparent folding back of 

 the edges on the concave surface (see a a, figure 2, and plate 4). This 

 infolding of the edges appears to have been before observed, and is quite 

 noticeable on the models of the meteorites from Puquios, Chile, and 

 Rancho de la Pila, Mexico. The regular curving of both surfaces of the 



Figure 2.— Diagrammatic Sketch of Meteorite's concave Surface. 



Designed to serve as a guide to plate 4. a a, a a, edges folded back ; b, deep pit caused by fusion 

 of schreibersite from the convex side (e of figure 1) ; cc, groove produced by a cold chisel ; d, d, 

 approximate margins of shallow thumb marks; e e, saw-cut extending partly through meteorite 

 (,/ of figure 1) ; kl and m n, saw-cuts. 



Algoma meteorite near the edges that are turned back favors the view 

 that this phenomenon is a result of slight bending of the marginal area 

 from the pressure of the compressed air on the front, the greater curva- 

 ture of the front (over that of the back) being ascribed to the erosive 

 action (see figure 3). 



THE MA ROINAL S URFA CE 



The marginal surface of the meteorite in all cases where the front does 

 not meet the back in a sharp line has a very hackly appearance and in- 

 dicates with little doubt a fracture surface. The more irregular contour 

 of the disc along its margin is for about half its length rounded off by 

 the curved front surface meeting the thumb-marked rear surface (see 

 plate 6, figure 1). Elsewhere, however, it has just the appearance of the 

 fracture surface of a malleable metal ruptured by tensile stresses, small 



