OF PRE-CAMBRIAN ROCKS 195 
phases until it is coarsely diabasic in texture. The gabbro is suc- 
ceeded by a thick ledge of white crystalline limestone, into which it 
tongues out with various small apophyses. The limestone is charged 
with streaks and bunches of silicates, which are contorted in so 
extreme a degree as to give truly remarkable results of pressure. 
Farther north quartzose gneisses and more gneissoid gabbro were 
traversed, the former greatly brecciated by faulting. From the 
railway the party climbed the hill to the old Cheever magnetite mine 
and its northward extension. The ore is found as an extended bed, 
sometimes double, and a half mile or more in length. It is contained 
in a gneiss, of granitic composition, which has gabbro a short dis- 
tance below and white crystalline hmestone a short distance above. 
Various opinions were advanced about the nature of the gneiss and 
the origin of the ore, sedimentary, intrusive, and contact views being 
all expressed without reaching unanimity. Returning by highway 
to Port Henry, the party visited the ophicalcite and white limestone 
quarries lying to the west of the road, and saw the latter rock much 
cut up by pinched pegmatites and basic stringers. 
The following day the committee drove to Mineville, six miles 
northwest, and on the way visited another quarry in white limestone 
and the abandoned Pilfershire magnetite mines. ‘The relations of 
the ore are much the same as at the Cheever, except that heavy drift 
so masks the exposures below the gneiss forming the wall-rock that 
their nature cannot be told. At Mineville the party visited ‘the 
great iron mines and studied the relations with the overlying acidic 
gneiss and the underlying, generally basic, hornblendic variety. At 
the belt of ore on Barton Hill it was shown that the underlying horn- 
blendic gneiss passed directly into gabbro. Various views were sug- 
gested for the formation of the ore, some favoring igneous segregation, 
others contact metamorphism, still others replacement or sedimenta- 
tion; but the problem was too obscure for agreement. In the evening 
the party was joined by C. R. Van Hise and A. E. Barlow. 
The following day the committee again went over the mines and 
examined the maps which were courteously shown by the chief 
engineer of the company, and later visited the outlying workings at 
Cook Shaft and Fisher Hill. The general feeling was that several 
successive processes might have been involved in producing the ore 
