Vol. 6, 1920 
PETROLOGY: G. P. MERRILL 
465 
(i. e., the porphyritic forms) in the Hght of knowledge gained from a study 
of terrestrial rocks, and as due to the gradual cooling of a melt of some 
considerable proportions to the point of crystallization of the phenocrysts, 
followed by an abrupt refrigeration resulting in the production of the 
glassy base. This crystallization and final cooling can scarcely have taken 
place after the drop or chondrule took on its present form. It would seem 
doubtful, also, if a melt which had cooled sufficiently to allow the formation 
of the phenocrysts would be sufficiently fluid to permit the formation of 
the spherical drops under any probable conditions. It is true the sep- 
arating out of the crystals would leave a magma more fusible than was 
the original, but it would still require an almost impossible condition to 
bring about a formation of the pellets as suggested. 
It is questionable further if conditions could have been such as to 
bring about a practically instantaneous, or rather simultaneous cooling 
throughout the entire chondrule. If cooling as an isolated drop of molten 
matter in which were floating the phenocrysts there would seemingly, 
almost for a certainty, be occasional indications of an earlier cooling on 
the outer surface manifested by the formation of the crust as already men- 
tioned, or conditions of strain or, possibly, signs of incipient crystalliza- 
tion. Such I do not find, the glass being uniform in physical properties 
and apparently in composition from centre to circumference of the chon- 
drule as it appears in the section. The same holds true of 
chondrules which are nearly or quite holocrystalline, like those of twinned 
pyroxene figured^^ by Tschermak from the Renazzo stone, or those from 
the CuUison, Kansas, stone described by myself .^^ In these cases one has 
to consider a much more uniform and gradual reduction of temperatures, 
such as would give the crystallographic forces full opportunity to perform 
their task. But were the material in the form of an isolated drop, and 
the forces given sufficient time the exterior form of the aggregate can 
scarcely be conceived as smoothly spherical, but must have been irregular 
with numerous projecting angles such as are frequently to be seen in 
spherical concretions of pyrite and marcasite. Such forms I do not find 
though I have sought for them diligently. In forms like figures 8, 9 and 10, 
it is, I think, self-evident that the original surface has been reduced 
by attrition, and whatever character it may have had has been lost. 
This same probability of reduction in size is suggested by the twinned 
pyroxene chondrules in the Parnallee stone. It is obvious that such can- 
not have resulted from the cooling and crystallization of a molten drop 
unless it were one of considerable greater magnitude. The occasional 
curved-enwrapping character of the outer crystals (fig. 11) can only 
be accounted for on the supposition of Tschermak and Daubree, already 
noted, that the material was somewhat soft and plastic. 
It is notable further, so far as my own observation goes and so far as 
