PERIDOTITE DIKES NEAR ITHACA, N. Y. 273 



cite), containing minute fibers of a clear, non-pleochroic mineral, 

 with high double refraction and nearly parallel extinction. This 

 was probably rutile. Similar alteration was frequently seen else- 

 where, and associated with the rutile was leucoxene. Scattered 

 through the sections is a light-yellow mineral, with a slightly lower 

 index of refraction than perofskite. This is probably picotite. It 

 is entirely isotropic between crossed nicols. 



Chlorite. — Chlorite is only important along the contact of the 

 dike with the shales, though it also occurs at the contact of mica and 

 serpentine. Doubtless the chlorite is the result of the action of 

 aluminous solutions on the ferro-magnesian mineral olivine. 



At right angles to the polarizer the fibers of chlorite are almost 

 colorless, having only a slight yellowish tinge. Parallel to the 

 polarizer the fibers have a bright green color. Basal sections usually 

 have a greenish color. They show a biaxial interference figure with 

 a small axial angle; and the sign is positive. The extinction angles 

 are very small. The fibers are often bent and broken, though few 

 show any crushing of pressure. The variety is probably prochlorite. 



Age of the dikes. — The age of the peridotite dikes is a difficult 

 question to solve. That they are younger than the Upper Devonian 

 rocks through which they have been intruded is perfectly evident. 

 During the field investigations certain facts were discovered which 

 may help to fix their age more definitely. The sedimentary rocks 

 have been folded into anticlines and synclines. The age of these 

 folds has been discussed by Mr. E. M. Kindle : 



It appears certain that the comparatively insignificant structural features 

 which have been described are of the same age and origin as the great open folds 

 of the northern Alleghenies. In the quadrangle cornering with the Watkins Glen 

 quadrangle on the southwest are folds whose arches, if restored, would rise 2,500 

 feet above their troughs. Less than twenty miles to the south of the Watkins 

 Glen quadrangle another great fold shows a crest of similar or greater elevation. 

 From theoretical considerations it would appear improbable that the effects of 

 the epirogenic forces which have developed structures of such magnitude should 

 terminate abruptly at the northern edge of the highly folded belt. Instead of 

 abrupt change from highly folded to monoclinal or nearly horizontal structure, 

 we find the mountain flexures subsiding gradually into the low, gentle swells which 

 have been described. This may be illustrated by a comparison of the maximum 

 dips exhibited by the anticlinal folds encountered between South Mountain in 

 Bradford County, Pennsylvania, and the southern end of Lake Seneca. Eighteen 



