1870.] T. S. IlLNT— ON GRANITIC ROCKS. 397 



some caseis, colorless vitreous quartz predomiQatcs greatly, and en- 

 closes crystals of milk-white orthoclase, often ir.odified, and from- 

 one to several inches in diameter. At other times pure vitreous 

 quartz forms one or both walls, or the center of the vein, or else is 

 arranged in bauds parallel with the sides of the vein, and some- 

 times a foot or more in thickness, alternating with similar bauds 

 consisting wholly or in great part of orthoclase, or of an admixture 

 of this mineral with quartz, having the peculiar structure of what 

 is called graphic granite, or else presenting a finely granitoid 

 mixture of the two minerals, with little or no mica, and with small 

 crystals of deep red garnet. Prisms of black tourmaline are also 

 met with in these veins, and more rarely beryl and even chryso 

 beryl. In the rock-cutting on the Lewiston railroad, just below 

 Topsham bridge over the Androscoggin, there is a fine exhibition 

 of these veins, which present alternate coarser and finer grained 

 layers, traversed by long spear-shaped crystals of dark mica pass- 

 ing from one layer to another. 



§ 17. A remarkable example of a vein of considerable dimen- 

 sions is seen in the feldspar-quarry in Topsham, which occurs in a 

 dark fine-grained friable micaceous schist. At the time of my 

 visit, in 1869, the limits of the vein were not seen, though large 

 quantities of white orthoclase and of vitreous quartz had already 

 been extracted. These were each nearly pure, and in alternate 

 bands, the quartz presenting drusy cavities lined with remarkable 

 tabular crystals. One band was made up in great part of large 

 crystals of mica, and portions of the vein consisted of a granular 

 saccharoidal feldspar. The famous locality of red, green and blue 

 tourmalines, with beryl, lepidolite, amblygonite, cassiterite, etc., 

 at Mount Mica in Paris, is a huge granitic vein, which, with many 

 others, is included in a dark colored very micaceous gneiss. 



§ 18. In AVestbrook numerous small veins of this kind, holding 

 coarsely lamellar orthoclase with black tourmaline and red garnet, 

 intersect strata of fine-grained whitish granitoid gneiss. In Wind- 

 ham the dark colored staurolite-bearing mica-schist of this series 

 is traversed by a granitic vein holding crystals of beryl. In 

 Lewiston a large vein of coarse graphic granite, holding black 

 tourmaline, and showinti; fine-iirrained bands, cuts a areat mass of 

 bluish gneissoid limestone, which forms an escarpment near the 

 railroad, about half a mile below the town. This limestone, which 

 dips eastward about 15'^, is interlaminated witli thin quartzite 

 beds, which are seen on weathered surfaces to be much contorted. 



Yol. Y. B ** Xo. 4. 



