6 Annals of the South African Museum. 



which are made up of rhombic pyroxenes. Of these two kinds are 

 distinguishable, those which in the middle are as dark as the matrix, 

 and those which pass from light grey to nearly white, stand out 

 more or less clearly, and give a spotted appearance to some parts of 

 the broken surface. The former predominate considerably. 



The shell is in most places so thin that in cross-section it is 

 scarcely noticeable, and seems more like a polish than an actual 

 layer; here and there, however, it reaches a thickness of \ mm., 

 and it is then differentiated clearly, though not over-sharply, from 

 the matrix. 



From the above description it will be seen that the St. Mark's 

 meteorite shows in various ways remarkable properties with regard 

 both to its shape and to the contour of its surface. Conspicuously 

 oriented meteorites are usually bell-shaped or conical or have at least 

 a convexity on the forward parts, but here the front is partly flat and 

 partly even hollowed, forms sharp edges with the sides, and is 

 roughly slab-shaped. A slab-shaped mass which enters the atmo- 

 sphere with a gi'eat velocity will undoubtedly make its passage with 

 one of its principal surfaces directed forward. But as this particular 

 stone is not bounded by two parallel surfaces, and there is therefore 

 an unequal distribution of mass, and as from the nature of the drift- 

 effect a high rate of rotation seems to me out of the question,* one 

 would not expect from the shape a priori stability of direction. As 

 a matter of fact, the meteorite cannot have altered its position 

 appreciably until the cessation of its initial velocity. The slab- 

 shaped meteoric iron of Algoma, described by Hobbs,| does not 

 altogether lend itself to direct comparison with this one, as the front 

 shows at least a slight convexity, and the surfaces of the slab, which 

 is only ^ cm. thick, are nearly parallel. 



We come then to the conclusion that in the case of perfectly 

 oriented meteorites a certain regularity of shape is indeed usual, but 

 not necessary, and we must take it for granted that, owing to the 

 great speed at which meteorites move, circumstances must play an 

 important role that we, in our limited investigations, can neither 

 reproduce nor study. 



Particularly striking is the slightness of the differences in the 

 sculpture of the back and of the front. On both surfaces, apart from 



* Had there been a high rate of rotation, then, in my opinion, a spiral arrange- 

 ment of the drift-effect, or at least a wearing off of the fine ribs through mechanical 

 corrosion would have become noticeable, v. E. Cohen, I.e., p. 173. 



t " Meteorite from Algoma, Wisconsin,' Bull. Gaol. Soc. of America, 1903, 

 xiv., 97-116, 



