﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 49 



garnet, fibrolite and epidote also occur. in large quantity. Occa- 

 sional streaks or masses are hornbiendic instead of micaceous. 

 These are interpreted as igneous injections. They are especially 

 abundant on Croton lake and near White Plains. 



It is essentially a quartz-mica schist. But it is almost everywhere 

 very coarse textured and hardly ever exhibits the fine grained, uni- 

 form structure of typical schist. Its abnormal make-up — the pre- 

 dominance of biotite and quartz — is the best defense for its petro- 

 graphic classification. The abundance of mica makes it a tough rock 

 but not very hard. The joints and fractures formed in later move- 

 ments are not healed and zones of bad shattering are susceptible to 

 considerable decay. These crushings are sufficiently common to en- 

 courage borings to tap their content of water for small family use 

 throughout Westchester county; but they do not represent large 

 circulation in any case. On the whole, the rock if fresh is good 

 and durable. It may, though rarely, carry considerable sulphide. 

 Practically all of the strictly original sedimentation marks are de- 

 stroyed by metamorphism. The formation has great thickness, but 

 because of the destruction of original bedding lines by recrystalli- 

 zation and additional complication by most complex folding, shear- 

 ing, crushing and faulting, the structure can not fully be unraveled 

 and the thickness can not be estimated with any approach to ac- 

 curacy of detail. But there is probably a thickness represented of 

 several thousand feet. 



(2) Inwood limestone or dolomite. This formation lies beneath 

 the Manhattan. It is everywhere coarsely crystalline either massive 

 or strongly bedded, often very impure with development of second- 

 ary (recrystallized) mica (phlogopite) and other silicates, espe- 

 cially tremolite. It is essentially a magnesian limestone or dolomite 

 in composition. There is an occasional quartzose bed in the midst 

 of the limestone as at East View. The upper beds are most charged 

 with mica and occasionally beds attacked by alteration have much 

 green, flaky chlorite. There are occasional interbeddings of lime- 

 stone and schist as a transition faeies. 



The coarser grades upon exposure to weathering readily yield by 

 disintegration to a lime (calcite) sand resembling: roug-hly an ordi- 

 nary sand in general appearance. At Inwood, the type locality, this 

 disintegration is so pronounced that great quantities are readily 

 shoveled up and used for various structural purposes in the place 

 of other sand. This dolomite is especially liable, as now shown by 

 extensive explorations, to serious decay to great depth. The under- 

 ground circulation seems to attack the micaceous beds with great 



