BLUE LIMESTONE. m 



third does not exceed 15 miles, while its greatest breadth is only 

 between 2 and 3 miles. 



The blue limestone, which we are now to describe, is found only 

 in the first and second of these chains of hills ; for in the third, the 

 aluminous schistus comes so near the surface, that several beds oc- 

 curring in the highest parts of the first two chains, are excluded. 

 The limestone, indeed, is thrust out, in not a few of the hills in the 

 first and second ranges ; as will appear from a more particular descrip- 

 tion of its localities.^ 



Though we speak of this bed as introducing us to the alum hills, 

 we do not mean to intimate, that it is usually the highest rock in 

 these hills, where their upper beds are entire; for there is often a 

 considerable thickness of strata over it : but it is the highest of the 

 rocks discernible on the coast, which we have identified among the 

 upper beds of these hills. When the great height and extent of the 

 first chain of the alum hills are congidered, it will not be deemed sur- 

 prising, that many beds exist in the interior, of which we find no 

 trace on the coast; and on the other hand, that there are some strata 

 on the coast, which have nothing corresponding with them in the 

 interior. It is well known, to all who have attended to the study of 

 geology, that beds which are of a great thickness in one part of the 

 strata, run out into a thin edge, and often entirely disappear, in other 

 parts; and that portions of the strata often change their character, 

 passing from one kind of rock into another. Instances of such disap- 

 pearance, or transition, have already been noticed, in describing the 

 last three members of our strata. 



The beds over the blue limestone, which, where that rock exists, 

 form the upper part of the first range of alum hills, and which de- 

 scending with their southern slopes, run under the back part of the 

 oolite hills, are different in different places. They consist of shale 

 and sandstone, generally siliceous ; and occasionally exhibit a mixture 



