No. 104.] 185 



differ from the schists east and nortli-east of the Highlands, in having 

 less quartz and more biotite and hornblende, and in being not so gene- 

 rally laminated in structure. From the mineral aggregation it is 

 evident that they contain more iron than the former and much less 

 silica in form of quartz. These differences, it should be here said, are 

 not apparent always in hand specimens. They come out when the 

 district is viewed more or less as a whole and by the field geologist 

 Avho has had some experience in the crystalline rocks to tlie north, in 

 the Highlands and in the country to the north-east of the Highlands. 

 On account of want of time it was not possible to trace out the out- 

 crops, marked by these diverse characters, or to determine how their 

 outcrops are related. 



From the occurrence of areas of grey, granitoid gneisses in the 

 Highhinds on the north, it is reasonable to assume that like belts or 

 isolated areas will be discovered in the Westchester county region 

 also — the southward continuation of the Highlands Archaean formation. 

 The existence of a main belt or range with its outliers on both the 

 north and the south, in Archaean time, is quite as probable as that of 

 like separate outcrops in the formations of subsequent eras. And ifc 

 appears to be so highly probable an hypothesis that it is presented in 

 this report as the one * est suited to meet-the facts and to explain the 

 structure of a part of Westchester county. What designation shall be 

 given to these typical schist formations is not so important. The 

 term Manhattan gneiss or series, proposed by Professor Hall in 

 , commends itself, coming from the typical localities on 

 New York Island and the adjacent parts of Westchester county. 



Note. — Reference should be made here to the work which has been done bj' Prof. Dana 

 on the geological structure of Westchester county and New York Island, particularly in 

 mapping thelimestone belts; iu showing the position of the strata in these belts and that 

 of the adjacent schists and gneisses; aod in the study and description of the rocks compos- 

 ing them. What Prof. Dana has done in the limestone out-crops adds very largely to our 

 knowledge of the district and is of great value in the study of the gneisses and associated 

 crystalline rock of the adjacent territory. A like carefully-executed survey of the gneisses 

 and schistose rocks will, it is believed, demonstrate that hypothesis which is set forth as 

 our working basis. 



[Aasem. Doc. No. 104.] 24 



