Augite from Vestivkis and Etna. 27 



to account for the different types of laccolithic differen- 

 tiation. This is based on fractional crystallization, 

 perhaps aided by convection currents, as Pirsson sup- 

 posed, the crystallization beginning at the rough walls, 

 and the crystals of this portion (in the present case mono- 

 mineralic) interfering with each other so as to produce a 

 granitoid textured rock. Crystallization of free floating 

 crystals (therefore euhedral) in the magma could, and 

 probably would, also go on simultaneously. The process 

 is analogous to the sIoav freezing of a bottle of salt solu- 

 tion, which begins at the walls, so that clear ice forms 

 above, at the sides, and at the bottom, leaving finally a 

 central core of highly concentrated solution. With the 

 more complex rock magmas the process would be 

 conceivably more complex than this, but the same general 

 principles would seem to apply. Unquestionably, the 

 influence of gravity might or would be felt, especially on 

 the loose floating crystals, but this would probably have 

 less or no effect on the wall accumulations. The process 

 is analogous to Daly's chilled border concept, but differs 

 from this in that, according to Pirs son's and my 

 hypothesis, the border crystallization product does not 

 represent the original magma, as conceived by Daly, but 

 would be an ^ ' extreme pole of differentation. ' ' 



On such a hypothesis the formation, either simulta- 

 neous or successive, of a granular pyroxenite, composed 

 of closely packed and adherent, anhedral crystals, and 

 free-floating, euhedral crystals, is readily understandable ; 

 more readily thus, it seems to me, than on a hypothesis 

 based purely on the influence of gravity. It also serves 

 to exjDlain, as that of gravity does not, such examples of 

 vertical, tubular differentiation as those of Magnet Cove 

 and Mount Johnston, Quebec, which are impliedly 

 regarded by Suess^^ as analogous to ^'piping" in a steel 

 ingot. 



But w^e are getting far away from our little augite 

 crystals. Let us pass on to those of a near-by volcano, 

 Etna. 



Augite of Monti Rossi, Etna. 



The loose crystals of augite that are found in abun- 

 dance in the ashes and tuffs of Monti Rossi, formed by 

 the eruption of 1669, and elsewhere around Etna, would 



^'Suess, The Face of the Earth, 4, 559, 1909. Cf. H. S. Washington, 

 Jour. Geol., 9, 607, 1901; F. D. Adams, Jour. GeoL, 11, 254, 1903. 



