JEFFERSON COUNTY. 
375 
It is in this compound rock that vve often find several interesting mineral species. Thus 
the sulphate of barytes, several secondary forms of carbonate of lime, sulphuret of iron and 
copper, flu or spar, carbonate of strontian, are 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 few unimportant exceptions. Grindstone island, for instance, 
is composed 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 small cavities, crystals of black tourmaline, with a 
few others less important, comprise all which 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 bearing along hard bodies, as 
rocks and gravel embraced 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 Jefferson, 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 Ave 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. 
Tliis rock is confined to the extreme northern border of the county, adjacent to St. Law¬ 
rence. It forms but in a feAV 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 polished 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 
