JEFFERSON COUNTY. :{75 



It is in this compound rock that we often find several intercslino; mineral species. Thus 

 the sulphate of barytes, several secondary forms of carbonate of lime, sulplmret of iron and 

 copper, fluorspar, carbonate of strontian, arc among the most common. 



The last mass of gneiss which I shall notice in this county, is at Alexandria bay. It resem- 

 bles that at Carthage, and passes into a hornblende rock, but in this mass we never find the pri- 

 mary limestone. From Alexandria it extends into the St. Lawrence river, and forms the base 

 of the Thousand islands, with a lew unimportant exceptions. Grindstone island, for instance, 

 is com])osed in part of gneiss or primary, and partly of the potsdam sandstone. Very few 

 minerals appear in this portion of the primary district : a few crystals of reddish feldspar, 

 laminated masses of the oxide of iron in siuall cavities, crystals of black tourmaline, with a 

 few others less important, comprise all wiiich have been observed. 



The masses of primary which have been described separately, probably belong to one con- 

 tinuous rock, upon which the potsdam and the succeeding limestones are deposited unconfor- 

 mably. The examination of these masses has produced the conviction, that wherever the 

 primary is exposed, it has resulted from a removal of the upper rocks, leaving the inferior 

 ones uncovered. This result originated from slight fractures and uplifts, by which the masses 

 were more or less broken up, and brought into a condition in which they could be acted upon 

 by certain physical forces, supposed to be currents of water bearuig along hard bodies, as 

 rocks and gravel erhbraced in masses of ice. 



All the ranges of gneiss which traverse this county from north to south, fall short of one 

 hundred and fifty feet above the level of the St. Lawrence, and are rarely over twenty or 

 twenty-five above the adjacent low grounds. 



In, or in connection with, the primary of Jefiferson, few plutonic rocks make their appear- 

 ance. At Theresa falls, dykes of white and greenish feldspar are intruded in the midst of a 

 confused mass of serpentine, primary limestone and gneiss. Milky quartz is also abundant 

 in seams at the same place ; and though we do not usually place this among intruded or vol- 

 canic rocks, yet here it bears some of the characters which are possessed by them. We 

 may, without doubt, attribute the origin of quartz frequently in primary rocks, to thermal 

 springs which have ceased to exist. 



Rensselaerite. 



This rock is confined to the extreme northern border of the county, adjacent to St. Law- 

 rence. It forms but in a few instances an extended mass, which is entitled to the character 

 of a rock. As a mineral, it exists in crystals near the village of Oxbow in Antwerp, in the 

 form of oblique rhombic prisms, and also in dark-colored radiating masses in limestone, and 

 associated with serpentine. 



Another locality of more importance, is near Butterfield lake, where it is in a large body. 

 From this locality I have received a fine polislied specimen which resembles the Italian mar- 

 ble ; the ground is dark, nearly black, and it is traversed by thin tortuous viens of a yellowish 

 color. The polished mass has a fine soft lustre, which imparts a beauty superior, if any 



