66 TACONIC SLATE, 



ceous slate. The latter is cvcn-gr.iincd, and finer in texture ; a fact which goes to prove 

 that the change is not due to nietamorphism, but to the character of the materials from 

 which it is formed. The color of these slates is mostly a pea-green, but they frequently 

 weather to a paler hue, and sometimes appear bleached or of a dirty grey upon the outside. 

 The coarser slates are always of a dirty greyish green, and though thin-bedded, never split 

 with an even surface. The fine-grained, on the contrary, are of brighter colors, and split 

 with a tolerably even surface, and hence have been used very extensively for flags. Other 

 portions of the rock are nearly black, passing into blue. This dark color is usually due to 

 the presence of sulphuret of iron, which, decomposing, imparts to the slate the black tinge 

 peculiar to one form of sulphur when liberated from its combination with iron. This fact 

 has led some geologists into error, by supposing that the dark color is due to the presence 

 of graphite ; and as this variety is often in proximity to the limestones or limestone shales, 

 it has led to the adoption of the opinion that by heat the limestone has been made to 3aeld 

 the carbon necessary to form the graphite ; and what has served to confirm the fallacy, is 

 the fact that the slate is often finely glazed by strong pressure, when raised from a hori- 

 zontal to its present inclined position. 



Tiie surface of this slate is often beautifully rippled, like many of the finer sandstones 

 in the New-York system, and still it retains the fine earthy texture of a sedimentary rock. 

 The principal change which it has suflfered, is the development of numerous natural joints, 

 by which the laminae separate into rhombic prisms with angles varying but little from 60° 

 and 120°. In very many places, it assumes that peculiar silvery greenish grey common 

 to the talcose schists. I have not observed any change in its state or condition, which can 

 be termed with propriety Tnetamorphic. It never loses its earthy texture, or never has 

 acquired characters which may not be ascribed simply to pressure, and the common mole- 

 cular attraction which all kinds of matter are subject to. The simple drying of the rock 

 has produced shrinkage cracks, which have been filled with calcareous spar or fibrous 

 limestone ; and wherever, by crushing and fracture, empty spaces were made, they too 

 have been filled with carbonate of lime or quartz, a result common to all rocks. 



The subordinate beds are, 



1. Coarse harsh sandstone with angular grains, imbued more or less with chloritic matter, and tra- 



versed very thickly with scams of quartz, by which the rock is divided or divisible into all kinds 

 of angular masses: their planes thickly set with imperfect crj-stals of quartz. 



2. Beds of grey sandstone with seams of quartz, but not very prominent. 



3. Green and black flinty slate, which breaks with a large conchoidal fracture. 



4. Blue compact limestone beds, and limestone breccia generally filled with sparry seams. Portions 



seem to be a regenerated rock from fragments of a pre-existing mass. Tliis is, however, quite 

 limited in extent. 



5. Roofing slate, of fine even texture and of a good quality. 



6. Red and chocolate-colored slates, usually fine-grained, but sometimes coarse and micaceous. 



7. Beds of grey siliceous limestone, whose characters approach nearly to the calciferous sandrock. 



It IS traversed also by seams of quartz and calcareous spar. 



