256 Williams — Cordact-riietaraovjjhisra near Peekshfll. 



schist to the massive rocks. Just at the contact the schistose 

 structure almost wholly disappears and the rock becomes hard 

 and massive. 



In order to understand these interesting alterations more 

 perfectly, we will examine each of the three above-mentioned 

 sections somewhat in detail. 



Section L~— At a point on the river shore just west of the 

 first brick-yard (Z on the map) mica schist is exposed at the 

 water's edge This stands nearly vertical and is finely banded 

 and very schistose, but is hardly at all crumpled or even bent. 

 A thin section, cut parallel to the bedding (No. 21), shows it 

 to be composed of quartz, biotite, muscovite and feldspar, (both 

 orthoclase and plagioclase) although the last-named mineral is 

 comparatively rare. Tourmaline in well-formed crystals is 

 common and zircon is also quite abundant, often occasioning 

 the pleochr/uc aureoles in the biotite (PL VI, ~Rg. 1). 



Not far from here, limestone is exposed and some of this 

 schist is so impregnated with calcium carbonate as to be over 

 half calcite. A specimen of this (No. 22) shows under the 

 microscope mainly quartz grains with some mica, both biotite 

 and muscovite. The quartz is filled with black granules and 

 contains an astonishing amount of rutile. This latter mineral 

 occurs in sharp crystals, mostly twinned, which range from 

 those thick enough to show a deep yellow color down to the 

 minutest which the microscope can reveal. 



No. 23 of the University, and K in Professor Dana's collec- 

 tion were obtained from a short distance north of those last 

 described. They are somewhat more contorted and contain 

 more abundant quartz nodules, but do not differ materially 

 from the less-altered mica schists except in the fact that gar- 

 net has been quite abundantly developed in them. The quartz, 

 both kinds of mica and the feldspar remain, but no tourmaline 

 was observed. No. 24, from a point somewhere farther north, 

 is a contorted muscovite-biotite-schist containing many quartz 

 " eyes " together with an abundance of large dodecahedral red 

 garnets. 



A slide in Professor Dana's collection marked M and made 

 from a specimen obtained on the line of section I, not far north 

 of the brick-sheds, is of remarkable beauty. It represents the 

 first phase of sillimanite-formation, where this mineral has 

 room for its complete development (PL YI, fig. 2). The base 

 of this rock seems to be quartz, triclinic feldspar and biotite. 

 The first constituent is filled with the minute, hair-like micro- 

 liths, which have been so often described but never yet deter- 

 mined. The biotite contains magnetite and an occasional 

 zircon. The new products are sillimauite, garnet and rutile. 

 The sillimanite forms thick brownish individuals which break 



