STOCKBRIDGE LIMESTONE. 79 
embracing therein all the limestones, good and bad, 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 subsecpiently 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 
slialy 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. 
