GABBEO AND NOEITE INTRUSIVES. 233 



GABBRO AND KORITE. 



PETROGRAPHICAL CHARACTERS. 



The gabbros and norites are holocrystalline rocks of moderately fine 

 to coarse grain. They show a considerable variation in texture. Some, the 

 finest-grained forms, possess a very good parallel textm-e (PI. XLII, figs. 

 A and B); others are noticeably porphyritic. A iew have poikilitic tex- 

 tures (PI. XLI, figs. A and B); less common is an approach to the ophitic 

 texture of the dolerites. Most common of all the rocks are the hypidio- 

 morphic granular ones (PI. XLIV, fig. A, and PI. XLIII, fig. A). 



The rocks vary from a light grayish-green color for some of the coarse- 

 grained ones, through darker greenish colored rocks to those of a dark 

 brownish or greenish-black color for the finest-grained forms. 



The important original mineral constituents are feldspar, mica, horn- 

 blende, pyroxene, and olivine. Apatite, sphene, zircon, rutile, octahedrite 

 (anatase), brookite ("?), and iron oxide occur as accessory minerals. White 

 and brown mica, chlorite, hornblende, talc, serjjentine, sphene, rutile, and 

 calcite occur as secondary minerals. 



Feldspar. — Botli plagloclasc and orthoclase are present in the gabbros and 

 norites. Plagioclase is by far the most important. It occurs normally in 

 the coarse-grained kinds of rock as broad tabular individuals. In the finer- 

 grained, especially the porphyritic and poikilitic facies, the feldspar sections 

 assume a broad, lath-shaped character. The sections show the character- 

 istic polysynthetic twinning. Twins, according to the albite, pericline, and 

 Carlsbad laws, are present, usually the albite and Carlsbad or the albite and 

 pericline being combined; in some cases all three occur together. Twin- 

 ning lamellae vary in breadth, but on the whole are moderately narrow. 

 Measurements made on the zone normal to 1 give equal extinction angles 

 against the twinning plane, which reach a maxim^^m of 34 degrees. The 

 feldsjDar is evidently labradorite. A zonal structure is noticeable, and is 

 especially shown by the alteration being more advanced in the centers of 

 the individuals. It is possible that the labradorite is accompanied by a 

 small amount of more acid feldspar, andesine in zonal g-rowth with it. 

 The alteration of the feldspar results in the production of the same sec- 

 ondary products formed from the slightly more acid feldspar of the diorites, 



