ELIZABETHTOWN AND PORT HENRY QUADRANGLES 1 27 



than the other eruptives, there is no doubt that eruptives which cut 

 the anorthosites are themselves later than the Grenville. 



When we come to the relations of the gabbros to the syenite series, 

 especially in the vicinity of Mineville, there is much obscurity. 

 When field work was first done by the writer in this area, the 

 syenites had not been recognized as such, and the rocks, which we 

 now believe to be embraced under them, were called hornblende- 

 gneiss or augite-gneiss. In these gneisses the gabbros when dis- 

 covered were believed to represent intrusive masses whose bound- 

 aries, because of lack of exposures, could not be delimited or 

 discovered. They were so involved with homblendic gneisses that 

 the latter were believed to have been derived from the gabbros. 

 But as already outlined very careful revision of the exposures along 

 Lake Champlain and on Barton hill, coupled with close microscopic 

 study of the drill cores, has shown the following relationships. 

 From the normal aggregate of feldspar and augite or hornblende, 

 or, less often, hypersthene, the feldspar predominating, the syen- 

 ites develop into basic bands in which the dark silicates are in ex- 

 cess. .Yet there is no marked difference in kind of minerals, only 

 a change in relative abundance. Garnets also appear, though not 

 frequently, and the rock becomes indistinguishable to the eye 

 from somewhat gneissoid derivatives of the gabbros. In the 

 writer's former paper ^ gabbros were mapped on Barton hill near 

 the Arch pit and again farther north near the Orchard pit. Between 

 these exposures of undoubted and typical gabbro'there is the stretch 

 of dark basic rock, which we also associate with the syenites. 

 Careful study has failed to show any recognizable contacts between 

 the two, or more than a gradual transition. One can not say where 

 the one ends and the other begins. Either the basic syenitic phases 

 develop at times into gabbros, possibly by infusion of limestones 

 from the Grenville or else the gabbros have been in some places 

 metamorphosed to hornblendic rocks indistinguishable under ordi- 

 nary examination from basic phases of the syenite. 



In the belief that the basic gneisses beneath the ore represented 

 the gabbro elsewhere seen in the foot wall in its massive form, the 

 writer developed in the former paper the interpretation of the ore 

 as a contact effect of the intrusive gabbro. In the present paper 

 the immediate wall rocks of the ore bodies have been consistently 

 described as members of the syenite series. With this change goes 

 inevitably a modification of the earlier interpretation of origin. 



1 Am. Inst. Min. Eng. Trans. 1897. 27:146. 



