102 NEW YORK STATE MUSEUM 



north rather coarse diabasic gabbro manifests itself. The contacts 

 of the gabbro on the gneiss are not specially easy to decipher but 

 they are believed to be those due to a succession of small block 

 faults, which produce repeated exposures and which may readily 

 lead the observer to infer the presence of more gabbro than is 

 necessarily existent. The latter may well constitute only a rela- 

 tively small dike or sheet. 



In the first observations of this cut, and in several subsequent 

 trips the writer gained the impression, not unnaturally, that a great 

 intrusive mass of gabbro had entered through or beneath the Gren- 

 ville series and that subsequent crushing and shearing had turned 

 its southern and northern portions into gneissoid derivatives, leav- 

 ing unsheared nuclei in the central parts ; ^ but it is a better inter- 

 pretation to infer an older intrusive sheet of syenite of acidic 

 and basic bands, and refer to this the apophyses of hornblendic 

 feldspathic rock which are so abundantly exhibited in the lime- 

 stones and then to believe that later came the gabbro which was 

 subsequently faulted in a way to give an undue impression of its 

 amount. 



The Crag Harbor ore then lies in the, syenitic gneiss in almost 

 the identical stratigraphic relations which are shown by the Cheever 

 and the Pilfershire bodies. 



One other possibility regarding the gabbro may be cited, which 

 is one suggested by other very obscure relations which prevail 

 between similar masses north of Cheever dock and on Barton hill, 

 Mineville. It is that by the involution or infusion of a limestone 

 mass the composition of the syenitic magma has been locally so 

 enriched with lime as to attain the composition of a gabbro and to 

 crystallize as such. On Barton hill it is almost if not quite im- 

 possible to detect the line where gabbro ends and basic syenite 

 begins, and it is none too easy along the lake front, but while this 

 explanation has been considered it has not been on the whole re- 

 garded with favor. 



There is still a third hypothesis which falls in line with the views 

 regarding the ores which have hitherto generally prevailed. It is, 

 that the gneisses immediately beneath the limestone are not of 

 igneous origin but are sediments in which the ores were deposited 

 along with other sedimentary materials. The ore may have been 

 magnetite sands, mechanically concentrated; or beds of brown 



1 Kemp, J. F. Gabbros on the Western Side of Lake Champlain. 

 Geol. Soc. Am. Bui. 1894. 5: 213, especially p. 221. 



