448 F. H. Lahee — Metamorphism and Geological Structure. 



Basic Intrusives. 



Description. — Dikes of minette have been found in southern 

 Conanicut Island, and of these there are seven instances.* Six 

 cut the fine greenish schists of the Beaver Tail Peninsula 

 (fig. 1, Loc. 23, 14 :D) and one cuts the pre-Carboniferous 

 Conanicut granite. 



These dikes consist of biotite and orthoclase, together with 

 some microcline, plagioclase, apatite, zircon, titanite, and the 

 secondary minerals, chlorite, calcite, epidote, limonite, and leu- 

 coxene.f The biotite, as noted by Collie and Pirsson, reveals 

 evidence of two generations. The older is represented by 

 large idiomorphic plates which are clouded — darkly near the 

 peripheries, but shading on inward — and which contain very 

 line rutile needles and leucoxene powder. The rutile needles 

 are especially abundant near the edges of the plates. They 

 are arranged in three directions, each at 60° to the others, in 

 planes parallel to the basal pinacoid of the mica. The leu- 

 coxene, while sparsely distributed, is more plentiful near the 

 borders of the mica flakes. 



Of the younger generation numerous smaller plates are seen 

 and also zones which encircle the earlier phenocrysts. This 

 later form, whether as separate crystals or as border zones, is 

 clear, greenish, and pleochroic, and contains no rutile nor leu- 

 coxene. When it occurs surrounding individuals of the first 

 generation, it is in optical orientation with them. Both types 

 have partly or wholly altered to chlorite. 



In the section investigated by the writer, there was no par- 

 allel orientation of the constituents, and evidences of crushing 

 were slight. In other specimens there is a distinct schistosity. 

 Pirsson:}; attributed the rutile inclusions in the biotite pheno- 

 crysts to the influence of the forces which produced this 

 schistosity ; but since the biotite crystals are surrounded by 

 zones of unrestrained mica and since the orthoclase grains may 

 abut against or enclose such coated phenocrysts with their 

 rutile needles, we conclude that the anomalous features 

 described for the biotite of the first generation were of mag- 

 matic origin. 



— delations of the minette to the Oarooniferous sediments. — 

 In some places the minette seems to have been intruded into 



* Certain ones of these dikes were described by the following writers in the 

 works cited: Foerste, A. F., in Geology of the Narragansett Basin, by 

 Shaler, N. S., Foerste, A. F., and Woodworth, J. B., U. S. G. S.,Monog. 

 xxxiii, 1899, p. 232. Pirsson, L. V., in his Geology and Petrography of 

 Conanicut Island, this Journal (3), xlvi, p. 363, 1893. Collie, G. L., in his 

 Geology of Conanicut Island, Trans. Wise. Acad., x, p. 199, 1894-1895. 

 Crosby, W. O., in his Contribution to the Geology of Newport Neck and 

 Conanicut Island, this Journal, iv, 230, 1897. 



f Pirsson gives a chemical analysis : op. cit., p. 375. 



iOp. cit., pp. 375-376. 



