408 G. p. MERRILL ON METAMORPHISM IN METEORITES 



ently so evident in the mesosiderite of Estherville, Iowa. In my recent 

 description of this stone^^ I have shown that in the process by which a 

 clastic mixture of pyroxene, olivine, and feldspar fragments was con- 

 verted into one essentially holocrystalline, the larger feldspar fragments 

 remain in all gradational forms from those unaltered and still retaining 

 their twin striations to those still only in part doubly refracting but quite 

 structureless, as seen under the microscope, and in some cases with an 

 unaltered nucleal center merging into a clear border, a plain product of 

 secondary fusion (see figure 5, plate 3). Facts like these and the occur- 

 rence in the Mocs stone of the maskelynite only in the fused crust should 

 seemingly be accepted as proving beyond a question the secondary origin 

 of the mineral through heat metamorphism, though that there are objec- 

 tions is noted later. 



The presence of the secondary form of phosphate, to which Dr. E. T. 

 Wherry has seen fit to give the name of merrillite,^* I consider equally 

 indicative of metamorphism by heat. It will be recalled that this form 

 of the mineral was first described by me in 1915^^ as an almost colorless 

 constituent occurring with irregular outlines in the ground-mass of vari- 

 ous stones and with optical properties too obscure for determination, but 

 the chemical nature of which was solved by micro-chemical methods. 

 The occurrence is similar to that of the maskelynite and leaves no ques- 

 tion as to its being one of the last minerals to solidify, even if not of 

 secondary origin (see figure 4, plate 3). Like the maskelynite, the min- 

 eral is found to be of almost universal occurrence in the white, gray, and 

 other chondrites of assumed metamorphic nature. 



Assuming, as apparently one must, the correctness of both the observa- 

 tions and conclusions drawn relative to the origin of the maskelynite, as. 

 given in the previous pages, the question naturally arises, Why are not 

 similar forms found in our terrestrial rocks? Assuredly conditions, as 

 along certain contact zones, must somewhere have been favorable; yet, so 

 far as the writer is aware, not a single example has been reported. 



Berwerth is inclined to draw a comparison between these forms and 

 some produced in terrestrial rocks through contact metamorphism, and 

 refers explicitly to the peculiar "geflossene'' form of pyroxene granules 

 in certain silicate rocks and the "Wachsthumformen" of the andalusite 

 in hornstone. I am not at present ready to comment on this. I can not, 

 however, agree with him when he says (page 645) : 



33 Proc. U. S. Nat. Mus., vol. 58, 1920, pp. 365-370. 

 3< American Mineralogist, vol. 2, no. 9, 1917. 



35 Proc. Nat. Acad, of Sciences, vol. 1, p. 302. See also Amor. .Tour. Sci., vol. 43, l'.>17. 

 pp. 322-324. 



