STOCKBRIDGE LIMESTONE. 79 



embracing therein all the limestones, good and had, in connection with the bed known as 

 marble. 



The principal differences in the beds comprised under this general name, are found in 

 the colors and texture of the rock. Of the colors, a small proportion are white and sac- 

 charoidal, fine and coarse, clouded and mottled with blue, dark or light, the latter forming 

 the dove-colored marbles. This embraces also the magnesian limestones, or the dolomites ; 

 inasmuch as the pure limestones pass into this species by imperceptible gradation. No real 

 difference is known as to position : both mineralogical species occupy the same range. 

 The coloring matter of the limestone is a substance derived from the slate, and which 

 seems to be only the matter of the slate in a state of fine division. These colors are not 

 known to change like those derived from some of the metallic oxides. The stains and 

 tarnishes which appear on some of the wrought marbles, are the effects of decomposed 

 sulphuret of iron, the presence of which is doubly injurious, by hindering the polish of the 

 material, and subsequently destroying the beauty of its color. The beds adjacent to the 

 slate are impure from the presence of this matter, which then appears only as a dark dirty 

 shaly limestone ; but many of the layers are largely contaminated with silex and masses 

 of quartz, which render the stone useless except for fences and the coarser materials of 

 construction. 



The siliceous limestones disintegrate rapidly even below the surface. Even the under- 

 ground ledges divide and separate into stones, which, when first exposed to the light and 

 air, are covered with a fine sandy coat. Probably the presence of magnesia facilitates the 

 disintegrating process, and assists materially the conversion of the rock into soil. 



This limestone is embraced in the Magnesian slate, and it is not possible to discover any 

 difference of the slate on either side of it : it is in fact one rock. The bed of limestone 

 commences with a few alternations of slate and impure limestone, till finally the beds of 

 the latter predominate. Their thickness is moderate at first, but increases towards the 

 central or middle part of the mass : they there become two feet thick. They are often 

 interlaminated with talcose matter, usually in distinct scales, and arranged so as to mark 

 the direction of the strata. Sometimes there is a mere sprinkling of it in scales the tenth 

 of an inch in diameter. 



The remarks which have now been made, apply to the principal deposit or main bed of 

 limestone traversing the Hoosic and Housatonic valleys in part, and thence passing south 

 and terminating on the Hudson river at Singsing in New-York. A few thinner beds run 

 parallel with the main one, resembling in this respect the relations of the Sparry limestone. 

 It appears from these considerations that at one period abundance of calcareous matter was 

 furnished from some source, probably the primary limestones of the Gneiss system, but 

 which, from some topographical change, ceased at another period to be furnished, though 

 at distant intervals it was supplied by a recurrence of the same causes. The smaller beds 

 of limestone run parallel with the larger, and all in the general strike of the system. 



