154 GEOLOGY OF THE SECOND DISTRICT. 



into Vermont and Canada, it maintains a remarkable uniformity in composition, structure and 

 other characters : the greatest change is in color ; being lighter, in some parts of the mass, 

 in some portions than others. The color sometimes is due to the presence of decomposing 

 sulphuret of iron. 



In speaking of the extent of this slaty rock, it has been questionable whether the range 

 farther cast, forming Saddle mountain, ought to be embraced under the same name. Without 

 pretending to decide confidently in regard to the similarity of the two, I am disposed at pre- 

 sent not to divide them. There is so much resemblance, that they may be considered as one 

 rock, without leading to serious error. With this disposition of the question, we shall be 

 obliged to give the rock a much gi-eater extent ; at the same time, it will be necessary that 

 we remember that they are separated by the rock termed the Stockbridge limestone. Both 

 masses of slate appear in mountain ridges, while the limestones form the valleys, and only 

 skirt them at their bases. 



3. Stockbridge Limestom'e. 



I have applied this name to all those varieties of limestone which are associated with those 

 masses that are usually known in market as the Stockbridge marbles. The only difference, 

 of course, which can exist, are found in the colors and texture, of each of which there are 

 numerous varieties, as snow white, and clouded with blue, either deeply, or merely a slight 

 mottling ; all of which vary in texture from very coarse to very fine. The differences in com- 

 position, taking in view the whole mass, are found to consist in the combination of magnesia, 

 forming dolomites, and a mixture (not a chemical combination) of silex. The coloring matter 

 appears to be derived from the slate which envelops it, and it probably consists of fine par- 

 ticles of slaty matter intimately mingled with the limestone. It is owing to this cause that 

 the colors are so permanent and fixed ; they never fade or appear to spread, or affect the stone 

 by becoming more diffused through it. The tarnishes and stains which sometimes appear, 

 are produced by pyrites, which decomposes slowly ; the oxide of iron spreading in the stone, 

 and imparting to it a dirty brownish hue. There are two kinds which are handsome when 

 first removed from the bed, but are truly of little value : the magnesian limestone, or dolo- 

 mite, which is tender and friable, and hence easily broken ; and those in which pyrites is 

 disseminated ; these latter, though perfectly fair when first raised from the quarry, will lose 

 their beauty by the decomposition of this substance. Those varieties in which silex is mixed, 

 are usually fine grained and subject to disintegration, whether exposed to the air or beneath 

 the surface. We often find a large bed of limestone beneath the soil, in which disintegration 

 has proceeded so far as to form a cobble of loose stones ; the process of disintegration having 

 penetrated first into the fissures and natural joints, until the bed is perfectly broken up. In 

 these cases, the pieces appear in some imitative shape, as the leg of an animal, or some 

 other natural object, and the surface is covered to some depth with fine particles which may 

 easily be fretted off. 



