GEOLOGY OF THE NEW YORK CITY AQUEDUCT 5 1 



the Cortlandt series to the very acid granites of Storm King moun- 

 tain or the granophyric pegmatites of North White Plains; and in 

 relative age they likewise vary from a period antedating the chief 

 early metamorphic transformation of the Grenville to Postman- 

 hattan time. But these clearly igneous types attain a considerable 

 prominence as separable units in the practical consideration of the 

 problems of the project and on that account the chief ones will be 

 more fully described under the next group. 



The older portion — the various schists, banded gneisses, quartz- 

 ites, quartzose gneisses, graphitic schists, and serpentinous and 

 tremolitic limestone, forming the complex through which and into 

 which the igneous masses have been injected — form together an 

 interbedded series that was originally a sedimentary group. There 

 is nothing known that is older in this region. Its characteristics and 

 relations mark it as in all probability the equivalent of the " Gren- 

 ville " of the Adirondacks and Canada. 



No single type and no single characteristic can be given as a 

 simple guide to the identification of this formation. The prevalence 

 of certain varieties or groups of these and the strongly banded 

 structure give a certain degree of character that forms a reason- 

 able working base. The formation includes banded granitic, horn- 

 blendic, micaceous and quartzose gneisses; mica, hornblende, 

 chlorite, quartz and epidote schists ; garnetiferous, pyritiferous, 

 graphitic, pyroxenic, tremolitic, and magnetitic schists and gneisses; 

 crystalline, tremolitic, and serpentinous limestones, aphi-dolomites, 

 serpentines and quartzites; pyrite, pyrohitite and magnetite de- 

 posits. This is the basal series. But it is complicated by a multi- 

 tude of bands of granitic and dioritic gneisses that represent 

 injections of igneous material at a time sufficiently remote to be 

 subjected to most of the early metamorphic modifications. The 

 equally abundant occurrences of quartz stringers and pegmatite 

 lenses though of later origin can not be separated from this com- 

 plex mass and the whole must be regarded as a physical unit. The 

 occurrence of interbedded limestones and quartzites together with 

 a variety of conformable schists and banded rocks, marks the 

 formation as essentially an old recrystallized sediment. 



No member of this older unit of the basal complex is sufficiently 

 prominent to indicate a great break or change up to the time of 

 the first great dynamic movements and igneous outbreaks. The 

 following comparatively constant members are sometimes persistent 

 enough to be considered formational units, but even more commonly 



