490 DIVISIONAL PLANES. 



crystalline quartz, is interesting. We cannot but regard it as another example and evidence of the abstrac- 

 tion of all other ingredients except the quartz, during the metamorphism of the rock, probably through the 

 aid of organic or carbonaceous matter. The idea that the pebbles were all originally hyaline quartz, is 

 highly improbable, since that is an unusual rock in quantity, and could hardly be abraded without a large 

 admixture of other rocks. The pebbles were probably of various kinds originally, but the quartz alone has 

 survived the decomposing agencies of metamorphism. 



3. Calciferous Clay Slate. Not unfrequently, especially where the clay slate approaches, or is interstrati- 

 fied with calciferous mica schist, the slate also becomes impregnated with carbonate of lime, and in some 

 cases the impure limestone forms beds in the slate. Where the slate gives a decided character to the rock, 

 it may perhaps be regarded as a variety of clay slate. 



4. Novaculite Slate. Novaculite differs so much from clay slate, that perhaps it should not be given as 

 a variety of that rock. Moreover it is not confined to clay slate in Vermont, but is often met with in 

 talcose schist. Indeed, it is rather a schist than a slate. But then its thickest beds are found in clay slate, 

 as in Guilford, where the bed is a quarter of a mile in thickness. We have also regarded a rock still more 

 extensive in the northern part of the State, as this rock. Its importance in an economical respect, since 

 good hones will doubtless be manufactured from it, entitles it to a description somewhere, and even of a 

 distinct color upon the Geological Map. We therefore place it as a variety of clay slate, and of talcose 

 schist also. 



Divisional Planes. 



The most obvious of these is slaty cleavage. In the western part of the State, where the slate is prob- 

 ably of an age somewhat different from that east of the mountains, we see occasionally cleavage planes 

 making an angle with the original stratification. Yet in the same quarries we find also joints and to 

 distinguish between the three kinds of structure is often quite difficult. In many places, however, we have 

 decided evidence that the slaty cleavage is coincident with the stratification. For beds of other rock are 

 present, and correspond in dip and strike with the cleavage planes. A good example of this sort may be 

 seen just within the limits of New York, in the slate quarry a little north of the village of Middle Gran- 

 ville. The cleavage there dips about 50 E., and the excavations have laid open a bed of sparry limestone, 

 four feet thick, which has the same dip and strike, but it shows no cleavage. In the clay slate east of the 

 Green Mountains, the interstratification of quartz, or mica schist, or novaculite, is not uncommon ; and we 

 have rarely, if ever, found the dip and strike of these beds different from the cleavage of the slate. 



We have intimated, in another place, that the strike of the cleavage planes, in the slate ranges east of 

 the mountains, does not probably differ much from the general strike of the formation. Approximately to 

 determine this point we have added together all the strikes given below, and find the averages to be as 

 follows. To these we have subjoined the general strike of the formations, as ascertained from our Geologi- 

 cal Map. Both are given from the true meridian : 



Range along Connecticut Eiver, average strike, N. 14 E. 



Strike of the formation, .... N. 13 E. 



Eange from Barnard to Newport, average strike, N. 14 E. 



Strike of the formation, N. 13 E. 



Eange running south from Troy, average strike, N. 8 E. 



Strike of the formation, . . . N. 8 E. 



The variation of the magnetic needle from the true north, according to the report of the Smithsonian 

 Institution, is 11 W.. in northeastern Vermont, and 9 W., in the southwest part ; the average, 10, wo 

 have used above. And though from the random manner in which the strikes have been taken, they can give 

 no very close approximation to the average strike of the formation, and though the observations on the 

 limited range of slate running through Troy are few, yet the near coincidences above given between the 

 average observed strikes and that from the map show a very close coincidence. 



One of our number (A. D. Hager) has attempted to measure the different dips and strikes of the differ- 

 ent structures in some of the slate quarries in Western Vermont, and we give the following examples from 

 his notes : 



