PART III. 



AZOIC ROCKS. 



BT C. H. HITCHCOCK. 



We now advance to a description of rocks in which not only all evidences of life are 

 gone, but, also, most of the original structure. That which they now exhibit has been in 

 a great measure superinduced by chemical and mechanical agencies. Stratification and 

 lamination appear to have been produced by original deposition in water. But cleavage, 

 which occurs in slate, is a tendency to split into thin plates, which are parallel, both in 

 dip and strike, over wide areas. The general opinion among geologists is, that it has 

 resulted from powerful pressure, which has extended the particles of the rock and made 

 them separate into layers. The strike and dip of these layers may, or may not, coincide 

 with the lamination and stratification. It would do so if the pressure by which cleavage is 

 produced were exerted at right angles to the stratification ; otherwise not, though we must 

 believe that chemical as well as mechanical agency has had something to do with cleavage. 

 The theory of superinduced structures, however, we shall consider in the fourth part of 

 our Report. 



To determine the coincidence or want of coincidence between stratification and cleavage 

 is one of the most difficult achievments of practical geology, and demands more time than 

 can generally be given to it in geological surveys in this country. We have not attempted 

 it to much extent in the slates of Vermont ; still less have we tried to draw a line of dis- 

 tinction between the foliation of the schists and stratification ; for this is a still more 

 difficult work. Hence the strikes and dips which we shall give are those of foliation and 

 cleavage. But we believe that these usually correspond essentially with the stratification. 

 In the fossiliferous clay slates of the western part of the State, as well as in the lime- 

 stones, we not unfrequently have found the cleavage planes making quite an angle with 

 those of original deposition ; but in all those belts of slate interstratified with the more 

 thoroughly metamorphosed schists, the two seem usually coincident. And we think that 

 in this case the plicating force, which had so much to do both with cleavage and plication, 

 acted nearly at right angles to the longer axis of the formations, which would bring the 

 two structures into coincidence. But more of this in another place. 



It seems to us, also, either that the term cleavage must be modified in its meaning, so 

 as to embrace not merely the indefinitely thin plates of slate, but the thicker plates which 

 we sometimes meet in limestone and quartz rock, and which cannot be referred to stratifi- 

 cation or joints. What- objection to saying that cleavage is the superinduced tendency in 

 rocks to split into plates, without limiting their thickness ? 



We ought to introduce here a definition of slates and schists in distinction from each 

 other. For geologists, after using these terms so long interchangeably, and without a 

 definite meaning, seem to be settling down upon a well-marked distinction. In a slaty 



