Gabbroid Rocks of Minnesota.— W'lncliell 359 
sists in supposing that the sihcoferrolyte is the result of 
exomorphic action of the gabbro upon a sedimentary rock 
originally composed of an impure iron ore, both quartzose 
and argilaceous. Its transformation has been effected not 
by means of pure igneous fusion, but by aqueo-igneous 
methods acting at a high temperature. In support of this 
hypothesis may be mentioned the frecjuence of the production 
of fayalite associated with free silica under the influence of 
pneumatolitic agents in cavities in certain granites of Ice- 
land,* and the great frequence of the same minerals in the 
lithophysai of the rhyolytes of the Yellowstone Park,t of 
Lipari,;*; of Glade creek, Wyoming, § and in those of the 
andesytes of Santorin,!! etc. 
Chaptek X, Resume and Conclusions. 
Comparative petrography. The rocks studied in this ar- 
ticle (except certain aberrant types) belong to a single petro- 
graphic family — that of the gabbros. 
The rock considered to represent the normal, central type 
of this family is the olivine gabbro, which is a dark gray, gran- 
ular aggregate of labradorite, pyroxene, and olivine, with more 
or less magnetite, and generally a few small crystals of apatite. 
It is nearly always in a remarkably fresh, unaltered condition. 
Its general structure is grandly massive with only occasionally 
a rude columnar arrangement on a very large scale ; any ap- 
proach to a bedded structure is very rare in the normal gabbro. 
Under the microscope the texture is found to be very coarsely 
granitoid with a tendency not uncommon to pass into a coarse 
ophitic texture. 
The variations which this rock undergoes are of three dis- 
tinct types : 
1. Variations due to conditions of solidification. 
2. Variations due to segregation, or original heterogene- 
ity of the magma. 
♦Delesse: Bull. Soc. G60I. France. II. X. p. 571. 
tiddings: 7th Ann. Rep. U. S. Geol. Survey. 1888. 
JIddings: Amer. Jour. Science. 1890. XL. p. 75. 
§Iddings: Amer. Jour. Science. 1891. XLI. p. 39. 
II A. Lacroix: C. Rendu. CXXV. 1897. p. 1189. 
