110 W. H. HOBBS METEORITE FROM ALGOMA, WISCONSIN 



vexl.y eroded front, with its radially spiral ridges and furrows, and the 

 hackly fracture surface of the margin. Of these surfaces the back most 

 resembles the surface of the greater number of meteorites, is presumptively 

 the one less affected by the flight through the aerosphere, and its main 

 features may be said, with little doubt, to date from the pre-serospheric 

 period of the body. The convex front was doubtless formed, so far at least 

 as its present surface is concerned, entirely within the aerospheric period, 

 while the hackly marginal surface doubtless represents a later pnase ot 

 the same period, since the erosion of the front ends not gradually, but 

 abruptly, at its edge. 



Emphasis should be laid on the fact that the back of the meteorite a^)- 

 pears to have come to us but little affected except for its scale of oxide, by 

 the flight of the body through the aerosphere. Formation of a scale of 

 this nature is so common on the back of oriented meteoric irons that 

 conditions in the wake of the meteorite must be assumed to be especially 

 favorable to such formation. 



Much has been written regarding the cause of the peculiar thumb 

 marks of meteorites. In the light of the recent investigation of Cham- 

 berlin,^ which shows that small meteoric bodies may be broken apart 

 through stresses induced by the near approach of larger bodies, a satis- 

 factor}^ explanation may perhaps be found. The ])robability also that 

 meteoric bodies travel in swarms, which has long been recognized, has 

 received -a valuable confirmation in the investigation of Hogbom.y He 

 has plotted the known falls of meteorites according to the days of the 

 )'^ear, and finds that those which fell at about the same time of the year 

 are remarkably similar in composition and texture. Goldschmidt J has 

 shown that the result of the impingement of ston}^ particles upon a rock 

 surface, as illustrated by the desert stones abraded by wind-blown sand, 

 is to produce a surface almost identical with that of certain thumb- 

 marked meteorites. 



From a consideration of the above evidence it would seem to the 

 writer that a possible explanation of the thumb-marked surface of many 

 meteorites may be found in the irregular surface produced by rupture 

 of a larger meteoric body subsequently abraded by the smaller meteoric 

 particles as it is drawn toward the earth out of the swarm in which it 

 was found. The greater number of the smaller bodies would doubtless 

 be vaporized, to reappear as the meteoric dust so commonly observed 

 in snow-covered regions. This hypothesis ascribes the main features of 



*0n a possible function of disruptive approach in the formation of meteorites, comets, and neb- 

 ulae. Jour. Geol., vol. 9 (1901), pp. 369-392. 



fEine meteojstatistische Studie. Bull. Geol. Inst. Univ. Upsula, vol. 5 (1901), pp. 132-143, pi. iv. 



lUeber Wustensteine und Meteoriten. Tschermak's min. u. petrog. Mitth., vol. 14 (1894), pp. 

 .V13, pis. i, ii. 



