MAGMATIC ALTERATION OF CERTAIN MINERALS 279 
the magma is erupted and consolidated would, to a certain extent, 
control the formation of augite on this hypothesis. We may 
therefore examine a few cases of variation of mineralogical com- 
position with geological occurrence which seem to bear out the 
view that many acid augite-andesites (and some of the more 
basic ones) are derived from hornblende-andesitic magmas of 
normal composition. 
In the andesites of the Auvergne, as recently described by 
Fouqué,’ the hornblende and biotite crystals are generally altered 
in the effusive lava streams, while in the very glassy effusives 
and in the dike rocks (which, according to Fouqué, have cooled 
quickly) they are fresh and augite is rare. It may be noted 
here, en passant, that in these rocks the presence of dusty inclu- 
sions in the apatite crystals seems to go hand in hand with the 
hornblende.’ 
In the augite-andesite flows of Mount Cimino, Deecke? 
observed that biotite predominates on the borders, while augite 
takes its place in the interior, of the streams. 
The difference observed by Hazard‘ in the stock and sheet 
basalts of the Lausitz is noteworthy. The latter are normal 
olivine basalts, with or without nepheline, while the former are 
hornblende basalts. His conclusions are that hornblende sepa- 
rated out as phenocrysts only in the eruptive canal, that it was 
gradually resorbed, changing into augitic substance, and that 
further on, when the rock spread out as a sheet, or was injected 
into cracks, olivine invariably separated out. ‘It is hence pos- 
sible to judge of the nature of the occurrence of our basalts 
(whether sheet or stock) from the constituents.” 
The explanation of such occurrences on the theory here 
suggested is quite simple. Hornblende and biotite, which 
have been formed at a depth, are preserved intact when the 
™Fouque, L’Etude des Feldspaths, Paris, 1894, 193-270. 
2 Cf. BLAAS, Jung. Erupt. Gest. Persiens. Min. Pet. Mitth., III., 476ff., 1880. Also, 
MERRILL, Erupt. Rocks fr. Montana. Proc. U.S. Nat. Mus., XVII., 642, 1895. 
3 DEECKE, Neu. Jahrb. B. Bd., VI., 231, 1889. 
4+ HAZARD, Min. Pet. Mitth., XIV., 303 ff., 1894. 
