60 KEMPS ORE DEPOSITS. 



schistose rocks has pinched them by small buckling folds and 

 shoved the ends slightly past each other in the process so familiar 

 in the production of reversed faults from monoclines. Sheared 

 granitic veins on a small scale are a not uncommon thing in areas of 

 schistose rocks, such as Manhattan Island, in the city limits of 

 New York, and suggest strongly this explanation. Should the 

 compression not go so far as to bring rupture of the bed, but only 

 a thickening by the formation of a sigmoid fold, it would occasion 

 an enlarged cross section, as has been suggested by B. T. Putnam 

 (Tenth Census, Vol. XV. 110) for the great magnetite ore body 

 at Mine 21, Mineville, in the Lake Champlain region. 



1.06.22. Quartz veins, often auriferous and of a lenticular 

 character, furnish another puzzling ore body. They are commonly 

 called segregated veins, and lie interbedded in slates or schistose 

 rocks. If in a pre-existing cavity, it must have been formed, 

 either by the opening of beds under compression, or by displace- 

 ment along the bedding, so that depressions came opposite each 

 other. Replaced lenses of limestone which had been squeezed into 

 this shape from an original, connected bed should also be instanced 

 as a possibility. The name " segregated " would imply a filling 

 by lateral secretion, but it is by no means impossible that solutions 

 have come from below. The veins are another attendant feature 

 on regional metamorphism, and as such deserve more investiga- 

 tion. 



1.06.23. The veins that contain cassiterite in many parts of 

 the world, and that yet have the mineralogical composition of 

 granite, are another product of metamorphic action, both contact 

 and regional. The gangue minerals, feldspar, quartz, and mica, 

 are quite characteristic of acid, igneous rocks, but the coarseness 

 of the crystallization in the comparatively narrow veins bars out a 

 true igneous form of origin. All our artificial methods of repro- 

 ducing these minerals lead us to infer. that the veins were filled at 

 a high temperature and pressure, therefore at considerable depths, 

 and through the aid of steam. Cassiterite has also been detected 

 in a few rare cases, under such circumstances that it seemed to be 

 an original mineral in igneous granite. It is probable, therefore, 

 that it may be an original and early crystallization from an igne- 

 ous magma, much as is magnetite. More observed cases would be 

 welcome as evidence. 



1.06.24. The great iron ore bodies of Vermilion Lake have 

 been referred by N. H. and H. V. Winchell, in the American 



