MINERALOGY AND PETROLOGY. 979 



Origin of gabbro.] 



rocks of the Animikie. Later, Prof. W. S. Bayley made an exhaustive study of the 

 field relations and of the petrology of the region of Pigeon point, and although his 

 first opinion was adverse to that view,* he finally adopted it in full.f He arrived at 

 the conclusion that the igneous quartz-porphyry of Pigeon point is derived from the 

 fusion of the acid elastics, and gives diagrams in which the porphyry intrudes upon 

 the quartzyte from which it is derived. He also shows that the same quartz-porphyry 

 in the same vicinity takes the form of quartz-keratophyre and red granite. This 

 view has already been presented for the origination of the granites of the Archean 

 (above) where the same transitions are manifest on a grand scale. 



Several rocks have been described from Pigeon point which illustrate the 

 manner in which the sedimentary rocks are converted to quartz-porphyry. It is 

 found that quartz is one of the first to be affected by metamorphism. It acquires a 

 bipyramidal shape (No. 264) and that almost cotemporaneously orthoclase crystals 

 form, the latter surrounding several clastic quartz grains in a poikilitic manner (No. 

 1842). Very soon all the quartz that remains is wholly recrystallized, much of it 

 apparently being removed in alkaline solution. More or less biotite and hornblende 

 appear, also nearly all the usual secondary minerals, which, however, here would be 

 called original under the prevalent usage, and in the classification given on page 969 

 would fall into the fifth class. 



There should be recorded here, perhaps, two or three caveats respecting some 

 of the acid igneous rocks, viz.: 1. It is not sufficiently known what is the source of 

 some of the "red rocks" and "augite syenytes" of the Keweenawan. Some of them 

 may come from deeper sources than the Animikie, even from the Archean, although 

 of the date of the Taconic. If they came from some deeper source they would be 

 necessarily dependent on the Archean clastic rocks (perhaps also on some of the 

 Archean igneous rocks) for their chemical composition. 2. It is not yet known how 

 much of the " red rock " series, such as that of the Misquah hills, may be due to a 

 reaction of the acid rocks on the basic of the Keweenawan. A widespread and 

 apparently rather profound endomorphism has affected some of the larger masses of 

 the basic Keweenawan, giving the impression of a later infusion of acid elements. 

 These questions must be left for future students to investigate, but it appears to the 

 writer that such silicified basic igneous rocks may owe their alteration to the fact of 

 submarine extrusion and to the oceanic precipitation of silica which would accom- 

 pany such conditions. 



2. The origin of the gabbro and its derivatives. 



It was after a conviction had been reached as to the origin of the Archean 

 granites that careful investigation revealed a similar train of evidence leading to the 



*American Journal of Science, 1889, vol. 37, p. 57. 

 ^Bulletin cix, U. S. Geol. Survey, 1803. 



