34 GEOLOGV OF THE SECOND DISTRICT. 



I should have been able to determine lliis question with gi-eater certainty, if the country had 

 Ijcen cleared of forests. As it is, there arc many impediments to the successful determination 

 of the question. Again : If grooves and scratches upon rocks are produced by boulders 

 moved along by water, we have another reason for not attributing their origin to the rock in 

 Essex ; for we have never observed maris of this kind in an east and west direction : they 

 preserve a north and south direction witii great constancy in the valleys of the St. Lawrence 

 and Cliamplain. Without pretending, however, to decide the question of the origin of these 

 boulders in St. Lawrence county, I would suggest whether it is not more probable that they 

 have been brought from Labrador, or some other region far to the nortli. 



Clat/ derived from the Hypersthene Rock. — I have already had occasion to remark, that 

 tliis rock change's from a dark smoke grey to a much liglitcr color, by exposm-e to the weather. 

 Being composed principally of a feldspar, it was not unexpected to find the rock forming, by 

 decomposition and disintegi-ation, a substance usually denominated porcelain clay. This re- 

 sult was realized in the discovery of clay in the vadey of the Adirondack above Lake Sand- 

 ford. It occurs in a small basin or meadow, through which a small creek flows which rises in 

 the adjacent mountain, and which is composed of this rock. The color of this clay is grey ; 

 it is highly refractory, but not infusible in a porcelain furnace. When burnt for brick, it 

 becomes a light yellowish-brown ; hence it appears to contain but little iron. 



This clay, on being moulded and placed in the foe, retains the original form of the mass, 

 and is not liable to fly and crack. It becomes a valuable article in this region, and. quite es- 

 sential in the construction of furnaces, chimneys, and other purposes in a manufacturing 

 establishment. 



Recapitulation. — I have been thus particular in describing the hypersthene rock and its 

 principal varieties. This course was called for ; inasmuch as it had not been regarded as an 

 Americcm rock, until after the survey of New- York was in progress. It is unquestionably one 

 of the most important prunary rocks in the State, whether wc take into view its extent, its 

 peculiar features, or its productiveness as an iron-bearing rock ; for this reason I will briefly 

 recapitulate some of its most striking characters. 



1. The rock is darker colored than the ordinary granites, but weathers to an ash grey. 



2. It is more or less crystalline ; as much so as the ordinary granites. 



3. It is composed essentially of only two substances, labradorite and hypersthene; the latter small in proportion 

 to the former ; hornblende often takes its i>lace. Epidoie, mica and quartz, so often or constantly present in other 

 granites, are very rare in this. 



4. Magnetic oxide of iron is also disseminated very frequently through the mass ; also garnet in grains or small 

 masses, scarcely ever in crystals : it is common near the junction of the rock with the ore beds. 



5. The rocks associated are sienite, and some obscure kinds of porphyry, which have been found only in rolled 

 masses on the banks of the rivers and streams. 



6. The rock has a jointed structure ; in addition to which, it is often traversed by segregated veins and cracks or 

 false joints ; the latter serving to divide the mass into wedge-form pieces, arc finally detached, and form the talus at 

 the base of the cliffs. 



7. It IS eminently an iron bearing rock ; bearing or embracing some of the largest and most important beds and 

 veins of the magnetic oxide yet discovered In the United States. 



8. The district of the hypersthene rock is alpine, the mountains being just upon the limit of perpetual frost. 



