CLAY SLATE. 489 



CLAY SLATE. 



BY E. HITCHCOCK, SENIOR. 



Whoever will compare a bed of clay where the layers have been deposited quietly above 

 one another, with the slates used for roofing, will notice a strong resemblance of form and 

 composition ; and he cannot but suspect that the latter has been derived from the former. 

 He can, if he will, trace out the steps of the process. Clay hardened by the sun and filled 

 with cracks, seems to be a sort of first step in the process. Among the newer sandstones 

 he will see similar layers, called shale, which is sometimes only a little harder than clay, and 

 seems to want only a smoother and glazed surface and induration to form clay slate. These 

 changes are produced in the shales by the more powerful influence of metamorphic 

 agencies, which generally also superinduce other divisional planes in the rock, such as 

 cleavage and joints. But cleavage planes in most of the clay slates of Vermont, coincide 

 essentially with those of deposition ; and the slaty layers seem to be mostly strata or 

 Iamina3 modified. If the modifying force were pressure, it seems to have operated to 

 convert the planes of lamination and stratification into those of cleavage, increasing the 

 number of the latter. 



Such being the origin of this rock, we might expect to find it in any formation where 

 the proper action has been sufficiently powerful to indurate, cleave, and glaze it, so as to 

 carry it beyond the condition of mere clay or shales. The same causes may make it also 

 fossiliferous or unfossiliferous. Under the fossiliferous rocks we have already expressed 

 the opinion that all the clay slate of Vermont will be found to contain fossils ; because we 

 have only to follow some of the beds a little beyond the State, on the line of strike, to 

 find them in New York and Canada. But since we have met with but few cases of this 

 sort in Vermont, and as the clay slate here is, at least in all the region east of the Green 

 Mountains, associated with unfossiliferous schists, we have thought it best to throw 

 together the most important particulars respecting this rock, under the azoic series, 

 though we shall try not to repeat what we have said respecting the clay slate associated 

 with fossiliferous rocks, in the west part of the State. 



Lithological Characters. 



These embrace the simple minerals that enter into the composition of the rock, their mode of arrangement, 

 and the varieties that result from the predominance of some of the ingredients, or of the admixture of for- 

 eign substances. 



1. The varieties in clay slate are few, unless we refer to color. The rock is usually simple and homoge- 

 neous, composed of finely comminuted, hardened clay. If it has a good deal of iron, and if this is passing to 

 the state of peroxyd, we shall have red slate, such as is quarried within the limits of New York, and occurs 

 in Vermont, as beneath the bridge over a river at Pawlet Center, where the junction of this red slate and 

 of talcoid schist is well exhibited. The red and gray slates, also, are often shown in the quarries in Castle- 

 ton, Fairhaven and West Haven. A greenish color also is not unfrequently seen, as in the Welsh quarries. 



2. On the margin of the quarries in Dummerston, we found loose fragments of slate filled with small 

 rounded pebbles of purely hyaline quartz. This forms a delicate conglomerate which still, however, retains 

 the slaty structure, though some of the pebbles are nearly two inches in diameter, and though the layers are 

 generally plicated like the schists. We did not find the rock in place, but cannot doubt that it occurs in 

 the vicinity of the quarries. 



The nodules of quartz in this rock are not in general flattened, nor is the laminated structure of the ce- 

 ment, properly speaking, cleavage, but more like foliation. But the fact that all the nodules are pure highly 



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