Ko. 200.] 
225 
must form a serious obstacle to its reduction, and greatly increase tlie 
trouble and expense of its manufacture into iron. Some of the ore beds 
of Essex are in this condition, but it is a matter of little moment at 
present, since there are others free from this objection, and are capable 
of furnishing sufficient ore to meet all the wants of capitalists. 
Limestone, ^ 
The limestone associated with the hypersthene rock, uniformly oc- 
curs in veins, is coarse granular, and is injured by the presence of nu- 
merous imbedded minerals, as the tabular spar, amphibole, pyroxine, 
idocrase, phosphate of lime, &c. The tabular spar is a mineral which 
resembles the coarse primitive limestone, and has actually been taken 
for it, blasted out, and put into a kiln and burnt, at an expense of some 
50 or 60 dollars. The relation of the limestone to the rock embracing 
it is such as to confirm the views already expressed of its origin, in the 
geology of the county of St. Lawrence. 
Phenomena of Dykes. 
The most remarkable geological phenomena of this region, are fur- 
nished by the dykes which traverse the hypersthene rock. A view of 
one of these is given in plate IV, and a distant view of another in 
plate VIII. on the right. 
The mineral character of the dykes may be reduced to four kinds: — 
1, a well characterized greenstone, with oval cavities, which have been 
filled with chlorite or carbonate of lime; 2, a compact amphibole, 
or hornblende; and 3, sienite, or a compound of feldspar and horn- 
blende; 4, a reddish porphyry, in which the crystals of feldspar are 
small and indistinct. They pursue an easterly and westerly course, and 
extend a great distance; and, indeed, I have not been able to ascertain 
their extent in a single instance. This is owing partly to the wooded 
state of the country. The largest of them is at Avalanche lake. It is 
80 feet wide, and cuts through Mount McMartin, nearly in its centre. 
The gorge formed by the breaking up of this dyke extends entirely or 
nearly to its summit. A portion of the northern face of the wall may 
be seen from lake Henderson, a distance of five or six miles. A view of 
this mountain is given in plate 7. This gorge exhibits, on a large scale, 
the powerful effects of frost and water, in breaking up the solid crust 
of the globe. In it are rocks from 50 to 100 feet in length, broken up 
from their original beds, and carried partly down the declivity; they lie 
in confusion in all directions, and constitute together a mass of ruins 
from the base of the mountain to its summit. On some of these detached 
[Assem. No. 200.] 25 
