54 MERRILL *^ 



As is well known, structures such as this stone possesses 

 have been accepted by Tschermak and others as indicative of 

 a tuffaceous origin — that is, they result not from the direct cool- 

 ing of a molten magma, but from the agglomeration of already 

 solidified particles, as is the case with volcanic tuffs. Others, 

 of whom Dr. M. E. Wadsworth is a prominent example, regard 

 them as the result of the hasty crystallization of an igneous 

 magma. ^ That many meteoric stones result from the cooling 

 and crystallization in place of igneous magmas is beyond ques- 

 tion ; yet there is an almost equal certainty that others are of 

 tuffaceous origin, though the nature of the evidence is not such 

 as to be fully appreciable except by petrographers. 



In 1888 I described a meteorite from the San Emigdio range 

 of California,- and announced my conclusion in favor of a tuf- 

 faceous origin. Nevertheless, as the stone was badly weathered 

 there has always been a question in my mind as to how much 

 of the apparent fragmental appearance was due to w^eathering 

 and how much was original. The absolutely fresh character 

 of the Allegan stone, which is of a surprisingly similar nature, 

 gives me an opportunity to reconsider the subject, and as it 

 happens, to confirm the views first expressed. 



The general structure of the Allegan stone can, I believe, be 

 accounted for only by regarding it as an agglomerate of chon- 

 drules imbedded in a fragmental groundmass or matrix, the ma- 

 terials for which were derived from the trituration of other 

 chondrules. 



One fact which, in m}- mind, has always mitigated against 

 the theory which would account for the peculiar structure of a 

 meteorite of this type on the assumption of hasty crystallization, 

 has been the complete absence of a glassy base in any but the 

 chondritic portions. Obviously, if the stone is a product of cry- 

 stallization in mass the chrondrules are products of the earliest 

 crystallization, and should, judged by the standard of terrestrial 

 petrography, be the most highly crystalline, while the base in 

 which they are imbedded might be glassy or crvstalline, accord- 



1 For a very clear exposition of Dr. Wadsworth's views see his Lithographic 

 Studies, pp. 1 06- 1 17. 



-Proc. U. S. Nat. Museum, Vol, XI, iSSS, pp. 161-167. 



