No. 200.] 
161 
rocks included under this general name, possesses a high geological in- 
terest, and in some of their forms the rocks are capable of important 
economical applications. Before entering upon the economical geology 
of these rocks, I may be permitted to make a single remark upon sci- 
entific geology. 
Mr. Bake well, the author of an excellent elementary work on geo- 
logy, has asserted, that the slate rocks of Great Britain have their clea- 
vage planes inclined at angles of about 60° and 120° with the planes of 
the strata. Prof. Eaton, and various others, from their observations, 
adopted BakewelPs views. My own observations led me for a long 
time to think this was true in the United States, as well as in Great 
Britain, and it was not until the past season, during which I have ex- 
amined with much minuteness the slate rocks of Columbia and Dutchess 
counties, that I have been convinced the rule is not so universal in its 
application as has been supposed. 
The local details, demonstrating that the inclination of the cleavage 
and strata planes of slate in Columbia and Dutchess counties vary 
through all angles from 0 to 90°, with the illustrative sections and views, 
will be given in the final report. 
A range of roof-slate extends from New-Lebanon through Canaan, 
Austerlitz, Hillsdale, Copake, Ancram, and Pulver's Corners in North- 
East. It is believed to be the same as that in w^hich the Hoosick quar- 
ries are located. Quarries of roof-slate have been opened in this range 
of rock in many places. The most important are those in New-Leba- 
non, about one and a half miles from the springs on the east face of the 
mountain. One belongs to Charles B. Gillet, the other, which is about 
a half or three-quarters of a mile north of the first, belongs to the Row- 
ley family. This last has been extensively worked, but for many years 
slate has been so cheap as to render the working of these quarries un- 
profitable. The slate is of good quality, (but softer than the Welch 
slate) and there is an inexhaustible quantity of it. 
At Hillsdale, a quarry of slate was wrought extensively some years 
ago, by Mr. Foster. The slate is of good quality where it is free from 
pyrites, but a large portion of it contains disseminated cubic crystals of 
this mineral. By exposure to the weather the crystals decompose and 
leave cubic cavities in the slate. The pyrites is an injury to the slate, 
not only by leaving cavities, through which leakage might take place, 
[Assem. No. 200.] 17 
