PETHOGUAI'MY. 333 



tive were found ;i still jjieater variety of inet;iiiu)ri)lii(; minerjils, iiicliuliiij; 

 isillimiiuite, cyauite, j>arnet, stanrolite, toiirmaliue, jjleonaste, cormidum, 

 margarite, ripidolite, nitile, spliene, ilnu'iiitc, zircon, magnetite, aiiiL;ite, 

 scapolite, zoisite, and cpidote. 



The limestone.s in tlie vicinity were by the same nj^encMes bli'ached 

 and fre(|uently rendered morecloscly crystalline, while lime-hearin^i; pyr- 

 oxenes and hornblendes, zoisite, sphene, and sca]»olit(^ ai'e <b',\'elope<l. 

 In the narrow dikes the nature of the eru[»ted rock was also moditied. 

 The iron and emery lu'ds along the southern and eastern i)ortions of 

 Cortland the area are regarded as a result of this same metamorphic 

 action ui)on pre-existing material, 



C/ohen* has described a case of conta(;t metamor})hism in which an 

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

 ing esseutially of quartz and minute colorless mica lamiiiic and clayey 

 matter colored by iron hydroxide, has been changed by contact with a 

 dike of diabase. Api)roaching the contactthe sandstone becomes green- 

 ish gray in color and the schistosity becomes obliterated. The green 

 color is due to the developuient of a greenish, strongly pleochroic and 

 doubly refracting chloritic mineral, which increases in (luantity as the 

 line of contact is approached. The stone also assumes a slightly higher 

 specific gravity and the fracture becomes choncoidal. With the in- 

 crease of the chloritic mineral there begin also to appear small Hecks of 

 a brown magnesia mica. The earthy (triiben) material disappears, as 

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

 parently gone to form the new ndca. As the zone of immediate contact 

 is approa(;hed, the rock becomes darker till finally grayish black, and the 

 choncoidal fracture more perfectly 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 horn- 

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

 no new minerals developed, although the structural arrangements are 

 somewhat changed. Analyses of samples in whi(!h the biotit;e lamina^, 

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

 shelly fracture and of the typical hornstone, showed that the 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 

 metamorphisms described by Stecherf 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, due presumably to the siliceous material dissolved 

 from the sandstone and shale. Olivine in (luite perfect crystals occurs 

 near the contact but gradually diminishes in quantity as oue recedes 

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

 on the supposition that the material of the dike cooled most quickly on 



* Neues Jalirb., 1887, Keil.-Band. 1. Holt, p. 251. 



tMip. ^. pet, MittlieU., IX, IJ., ii, uml in, Heft, pp. l45-'^0.5, 



