184 [Assembly 



cal history, and the term Azoic is as applicable to large areas of un- 

 doubted newer formations as to the Highlands rocks, if the absence 

 of life remains be the basis of our nomenclature. Lastly the possibility 

 of future discoveries of the slightest traces of even th lowliest forms 

 of life renders the continuance of Azoic doubtful. Archcean is not 

 open to these objections, including, as it docs, the earliest rocks, or 

 original crust of the earth and the first sediments deposited in that 

 "■ era in which appeared the earliest and simplest forms of animals." * 

 Adopting the term Archcean, the possibility of a subdivision upon 

 lithological grounds has been pointed out in the statement upon 

 structure.! The more massive and unstratified out-crops, or granit- 

 oid gneissic areas constitute the older central masses about which the 

 more distinctly bedded and schistose rocks have been deposited. And 

 they belong to two well-defined horizons or periods of Archfean time. 

 At present there is no evidence from any remains of life to separate 

 the one from the other. They are distinguished by their diverse lith- 

 ological characters and geographical position. It may be said here 

 that the rocks of the New York Highlands resemble closely the typi- 

 cal gneisses and other crystalline rocks of the Laurentian of Canada, 

 but this resemblance, of itself, appears insufficient to decide the fact of 

 exact equivalency of horizon in the case of so widely-separated forma- 

 tions. 



The identification of a Huronian group "by means of rock characters 

 has not been made out, nor do there appear to be, so far as the present 

 reconnaissance goes, any out-crops which can be thus recognized as 

 corresponding to the tyjjical Huronian rocks. Provisionally, the 

 formation in the Highlands, as outlined in the preceding sections, is 

 designated as Arclueaoi. It may be Laurentian also.§ 



The reconnaissance in the country south of the Highlands and in 

 Westchester county shows the existence of a great variety of crystal- 

 line rocks. Aside from the out-crop of the Cortland series of Prof. 

 Dana, and the limestones there appear to be two great classes of out- 

 crops, as in the Highlands, but not counterparts of one another in 

 all respects. The Highlands type of a grey, massive, granitoid gneiss 

 approaching a granulyte, is recognized but not developed over sa 

 wide belts, apparently, as to the north, in the Archaean district. || 

 Far more common is the other class, in which are here included the 

 micaceous gneisses and mica schist marked by the presence of biotite 

 and less frequently by garnets, and the hornblende schists which con- 

 tain biotite also. These micaceous and hornblendic rocks are dark- 

 colored, and are schistose in structure. In stratification the bedding 

 of the latter is thin and more contorted in strike. To the ordinary 

 observer they look possibly more like the common fragmental rocks 

 than do the more massive grey, granitoid gneisses. They seem to 



* Manual of Geology, by Jas. D. Dana, 1874, p. . See, also, Am. Jour. 0/ Science {^)> 

 XXVIII, pp. 31:3-314. 



+ Pages of this report. 



% It is proper'to state iu this connectiou, that in the absence of all palaeontological evi- 

 dences of age, this recourse to the nature of the rocks is not considered as altogether and 

 absolutely scientific, since lithological resemblances cannot be regarded as conclusive 

 proofs ot geological age. They indicate like conditions, and these conditions ivhen found 

 prevailing over Wide areas of out-crops, which are overlain by rocks of known horizons, 

 may be suggestive of equivalency of age when taken in connection with the I'elation to 

 adjacent fossiliferons formations. 



Ij See page of this report. 



