Kemp — Geology, of Essex County. 



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typical cOarse, massive, bluish black anorthosite is to be had in endless 

 amount. It is a beautiful rock and could not fail to impress any geologist as 

 a most attractive object of study. The dark color of the fresh rock becomes 

 a pale blue or grey on exposure. Its water-worn pebbles display at times the 

 characteristic play of colors, and suggested the name Opalescent for the 

 creek or small river that drains lake Colden. These rocks extend in typical 

 development to the northeast, and are found in all the peaks around lake 

 Colden. They form the walls of Indian pass and are present around the 

 Preston ponds, but how far they extend to the west I can not state from 

 personal observation. 



Lakes Sanford and Henderson are classic ground in connection with the 

 early metallurgy of iron in this country, and as the result of Professor E. 

 Emmons 1 Report on the Geology of the Fourth District, in 1841, the 

 knowledge of the enormous deposits was spread abroad and no doubt in part 

 through the influence of this report the historic old furnace was con- 

 structed. More extended mention of it is made later on, but the purely 

 scientific question of the relations of the ores to the wall-rock may be 

 here discussed. The question is a quite different one from that of 

 the ordinary lenticular deposits of magnetite, parallel to the laminations 

 of gneisses. The ores are titaniferous in varying amounts up to a max- 

 imum of nineteen per cent. The walls are a perfectly massive plutonic 

 rock, consisting of little else than labradorite. At the mill-dam opening, 

 where the wall-rocks are the best exposed of any, and on each side of 

 twelve to fifteen feet of pure ore, they are of exactly this tvpe and 

 are perfectly unmetamorpbosed. In the river bed near the iron dam or 

 ledge of ore that makes a little reef, and at the mouth of Calamity 

 brook, the ore is mixed through the rock in clots of all sizes, from that of a 

 nut, upward. In the ore itself are found large plagioclase crystals of dark 

 green color because so charged with dusty augites as to be opaque, even in 

 thin section, and around the edge of each feldspar and separating it from 

 contact with the ore is a rim of brown hornblende and biotite, up to an eighth 

 of an inch (three millimeters) thick. These plagioclase crystals are almost as 

 large as a man's hand as a maximum, but they are usually one or two square 

 inches in area. The)- are not strained, so far as my observation goes, and they 

 show the twinning striae sometimes even to the unaided eye. The best place 

 to study them is at the Sanford bed in the prospect opened up on the hillside 

 about a mile from the lake, approximately at No. 44. There seems no escape 

 from considering these ores as of true igneous origin, separated from a cooling 



