64 I*)'of. Dana — MetamorpMsm of Massive Crystalline Rocks. 



east of limestone No. 4 (see map), and also north of this limestone 

 in the field west of the road, where it is conformable with the 

 limestone. 



E. The Ch-ysolitic Bodes. — The chrysolitic rocks of the region 

 have no resemblance in aspect to ordinaiy chrysolitic volcanic or 

 igneous rocks. The kinds are (1) chrysolitic hornblendite, (2) chryso- 

 litic pyroxenite, (3) chrysolitic norite ; and these graduate not only 

 togethei', but also into a rock, in which chrysolite is the chief con- 

 stituent. They are black or brownish-black rocks, and are mostly 

 coarsely crystallized, the hornblende crystals being often an inch 

 in length or breadth ; and the chrysolite is in grains of various 

 irregular forms and sizes, distributed through these crystals as well 

 as among them, and not in well-formed crystals. In general, they 

 contain but little felspar, and this is triclinic ; and a variety inter- 

 mediate between the hornblendite and pyroxenite is common. 

 They contain occasionally black mica, but no quartz. The chrysolite 

 is more or less altered, as is shown (when examined in thin slices) 

 by the bordering and intersecting bands of magnetite and viridite, 

 and in some cases it appears to be changed to serpentine.^ 



These rocks are largely exposed along the western half of the 

 north side of Stony Point, west of the boat pier (the area is lettered 

 z ?J on the map), and over Montrose Point, as well as in its vicinity; 

 at which places they are associated with norite and other rocks of 

 the series. They also outcrop in eastern and southern Cortland. 

 The most eastern locality observed is within half a mile of the 

 eastern border of the town, near the middle of the three " emery " 

 mines referred to beyond, and the most southerly, a short distance 

 east of Croton, within half a mile of Croton River. 



The chrysolitic rocks are the most decomposable of the series, 

 and wherever the brown-black ledges are crumbling in an extra- 

 ordinary way, and making a profusion of brown sand or brown or 

 red earth, the presence of chrysolite may be suspected. 



F. Iron and Emery Mines. — This Cortland region has its mines of 

 magnetite, some of which are also mines of emery. The containing 

 rock is either norite, diorite, or soda-granite, and even chrysolitic 

 rocks are sometimes near by. The iron ore has been found at 

 several points within a mile north and north-east of Cruger's, and 

 also three or four miles distant in the eastern part of the township 

 of Cortland ; but the amount appears to be small, and no workings 

 have yet proved profitable. The ore is commonly very chloritic, 

 and contains less magnetite than the appearance and weight seem to 

 indicate. 



At a mine three-fourths of a mile north of Cruger's (at u, on 



' These chrysolitic rocks usually have, on a fresh fracture, the cleavage surfaces of 

 the hornblende or augite spotted with chrysolite ; but the presence of chrysoKte, 

 however abundant, cannot be made certain without slicing for microscopic examina- 

 tion, since the chrysolite is slightly altered externally, and such spots on hornblende 

 crystals may be due to small imbedded crystals of augite. If the cleavage of the 

 hornblende has an unbroken surface, it is probable that the rocks contain no chryso- 

 lite. The hornblendite has much stronger lustre than the pyroxenite. 



