PETROGRAPHY. 333 



tive were Ibiiiul a still greater variety of metamorpliic miiieraKs, including 

 silliuuinite, cyauite, garnet, staurolite, tourmaline, pleonaste, ('oriinduin, 

 nuirgarite, ripidolite, rutile, spliene, ihnenite, zircon, nuignetite, augite, 

 scapolite, zoisite, and epidote. 



The limestones in tlie vicinity were by the sanu^ agencies bleached 

 and frequently rendeied nioreclosely crystalline, while lime- bearing pyr- 

 oxenes and hornblendes, zoisite, sphene, and scai)olite are dev(doped. 

 In the narrow dikes the nature of the emptied rock was also modified. 

 The iron and emery beds along the southern and eastern i)ortions of 

 Cortland the area are legarded as a result of this same metauiorphic 

 action uj)(»n i>rc-existing material. 



ColuMi* has described a case »)f contact metamorphism in which an 

 ochre yellow, line-grained, and imperfectly schistose sandstone, consist- 

 ing essentially ol"<iuartz and minute colorless mica lamina' and clayey 

 mattei- cohncd by iron hydroxide, has been changed ))y contact with a 

 dike of diabase. iVpproaching the contactthe sandstone becomes green- 

 ish gray in color and the sciiistosity Ix'comes obliterated. The green 

 color is due to the deveIoi)ment of a greenish, strongly ])leoclir()ic and 

 doubly refrac^ting chloritic mineral, which increases in (juantity as the 

 line of contact is ai»proached. The stone also assumes a slightly higher 

 si)ecitic gravity and the fractuie becomes choncoidal. With the, in- 

 crease of the ('hloritic mineral there begin also to aj)pear small tlecks of 

 a brown magnesia mica. The earthy (triiben) material disap[)ears, as 

 does also the small amount of calcareous matter observed, having ap- 

 parently gone to loiin t he nc^w mica. As the zone of immediate contact 

 is api)roached, the rock becomes darker till finally grayish black, and the 

 choncoidal fraciture more i)erfectly developed. The microscope shows 

 it to still consist of the secondary chlorite and brown mica together 

 with the original constituents. At contact the stone is a typical horu- 

 fels or lydiaii stone of a clear black color and shelly fracture, but with 

 no new minerals develoi)ed, although the structural arrangements are 

 somewhat changed. Analyses of samj)les in which the biotite lamina' 

 were beginning to be developed, of the grayish black variety with 

 shelly fra(!ture and of the ty[)ical hornstone, showed that tlie composi- 

 tion had in all cases remained practically unchanged, that the changes 

 were not due to any addition of material from the dike. In contact 

 metaniorphisms described by Stechert the sandstones and shales have 

 have exercised a very considerable influence upon the olivine diabases 

 cutting them. The material of the dikes was found to be more acid 

 near the contact line, <bie presumably to the siliceous material dissolved 

 from the sandstone and shale. Olivine in (juite [)erfect crystals occurs 

 near the contact but gradually diminishes in (piantity as one recedes 

 till it is wholly lacking in the center. This is accounted for by Steelier 

 on the su[)i)osition that the material of the dike cooled inost (juickly ou 



* Nones Ja'brb., 1887, Beil.-Baud. 1. Holt, p. 251. 



tMiu. u. i)et. Mittlicil., IX. li., ii. ami in. Helt, i)i). 115-20.'}. 



