i-50 



NEW YORK STATE MUSEUM 



This complex of gneiss and granite makes up the mass of 

 Dannemora mountain and forms the hills in Dannemora and 

 Beekinantown in the southwest corner of the Mooers sheet. 



As has been stated in previous reports, if there is any forma- 

 tion in the northern Adirondack region which is the equivalent 

 of the Ottawa gneiss of Canada, or which may be of Archaean 

 age in the restricted sense in which that term is now employed 

 by the United States geological survey, it is this Dannemora 

 formation. Unfortunately in the district in Clinton and Franklin 

 counties in which it prevails, the Grenville series is practically 

 absent, and a rather careful survey of the area has failed to dis- 

 close any exposures which throw any light on the question. The 

 problem is much complicated by the intimate association of very 

 similar gneisses with the undoubted Grenville rocks in the 

 western Adirondacks, where, if anywhere, there is a prospect of 

 settling this question. 



On Rand hill, specially at its eastern edge, is considerable 

 gneiss which is cut through and through by the gabbro, with un- 

 mistakable irruptive contacts, showing that the gneiss is the 

 older rock. 1 The gneiss that appears on the west side of the hill 

 is mainly of the black dioritic variety, though there is much of 

 the red gneiss also. It, seems totally unaffected by the gabbro, 

 and there is thought to be a fault between, though the evidence 

 of this is not decisive. On the east the gneiss is best shown along 

 the cliff face and along Mead brook. Here it is all cut up by the 

 gabbro and differs quite materially in character from that on the 

 west. Nowhere else in the Adirondack region has the writer seen 

 such contacts, nor has he met elsewhere a gneiss precisely similar 

 to this. 



It is for the most part a well foliated, black and red or black 

 and gray gneiss, quite like some intermediate varieties of the or- 

 dinary gneiss. But often, specially near the gabbro contact, it 

 is hard and flinty and breaks with a conchoidal fracture. In 



1 Rand hill, as here used, applies to the whole area on which the gabbro 

 outcrops, the low north end as well as the summit; locally it is usually 

 restricted to the latter. 



