BIOTITEGKxVNITE, 321 



CORDIERITE-GRANITITE. 



At Briintiold, in the roadside near the iiortli line of the town, is a 

 coarse granitic rock made up ahnost wholly ot" granular feldspar, in which 

 large, rounded crystals of the same are embedded. In both forms the feld- 

 spar is largely transparent. Thin films of l)i<>tite, mostly changed to 

 chlorite, are shot through with tufts of tine iibrolite. Clai-nets and flakes of 

 graphite are irregularh' disseminated. Large, granular masses of nearly 

 black, fresh cordierite occur, which are at times amethystine. 



The feldspar proves to be almost wholly microcline, with finest micro- 

 perthitic structure (which is the cause of the moonstone luster) ;nid with 

 crushed borders, and it contains unusually large and well-defined zircons. 

 The quartz contains many long, curved rutile needles. 



The cordierite is exceptionally fresh; rarely there spreads in fissures 

 a delicate, feathery growth of limonite, and the mineral is altered for a 

 small distance into a yellow, serpentine-like mass having aggregate polar- 

 ization. It contains in great numbers regular hexagonal plates of hematite, 

 placed in two planes at right angles to each other. 



Interposed laminae occur at times in twin positions. (See fig. 2, PI. III.) 

 There are two sets, making an angle of about 61° with each other. TheA' 

 are long, rigidly sti'aight and parallel, narrow plates, sometimes slightly 

 tapering or truncated at the end by an oljlique plane. 



Sometimes a broad nntwinned area sends a great number of these thin 

 bands far into the untwinned area of another crj^stal. 



At times the bands interlace and include many diamond-sha|)ed fields 

 of the host. riiey are unlike plagioclase bands in that they are sur- 

 rounded in polarized light by a white band. This is because the plane of 

 boundary runs obliquely to the plane of the section, and the complementary 

 colors of two parts neutralize each other. The fibrolite runs uj), branching 

 and rebranching like a plant, and at the end of each branch bright-green 

 plates of chlorite are attached like leaves. In some cases it seems as if the 

 square prism of the fibrolite were changed to chlorite. 



In fig. 2 of PI. Ill the unshaded portion shows the axial figure of tlie 

 fii'st crystal (I) eccentrically as indicated. This crystal was large, and 

 from a second, smaller crystalline portion (II) blades generally rigidly 

 straight and Avith straight boimdaries were sent out into (I). These plates 



MON XXIX 21 



