1887. | NEW YORK ACADEMY OF SCIENCES. 25 
hardness, great weight, and high compressive strength of this 
stone are the natural results. At Lee, Massachusetts, toward 
the southern end of the belt, the beds of fine white to blue-gray 
marble, though but gently inclined, present similar physical 
characteristics, a sparse but general distribution of magnesian 
mica, tremolite, and minute particles of an unstable form of 
pyrite, and a more marked inclination to laminated structure. 
This lamination becomes a more decided feature in the gray 
limestone of Stockbridge and the marbles of Sheffield and Canaan 
to the south. 
To these well-known characteristics I may add the fact of the 
peculiar mode of disintegration which affects this rock, under 
the influence of the weather and percolating moisture, as shown 
in the sides of quarriesand outcrops throughout the distribution 
of the sheet, from Vermont to Connecticut. The material be- 
comes finely cracked up by a network of minute irregular crey- 
ices, but is also often seamed by long cracks parallel to the 
planes of bedding, especially within a few inches of the natural 
division-surfaces. 
The object of this review has been to impress the conclusion 
of the general lithological uniformity of this limestone-sheet, 
with the unimportant exception of those subordinate character- 
istics which depend mainly upon local variations in mode and 
degree of crystallization. On the other hand, however, it has 
been found that a highly magnesian composition, approaching 
more or less closely to that of a true dolomyte, belongs to certain 
beds, usually accepted as portions of the same continuous sheet, 
which occur mostly on each side of the central band and around 
its southerly termination. There is sufficient evidence, in my 
opinion, to lead us to recall the view, at one time suggested by 
Prof. Dana, and to consider at least many of these dark-colored 
beds, so highly magnesian as to be entirely unfit for burning to 
lime, not as parts of the main limestone-sheet, but members of 
a series of intermittent occurrence lying above it. Their mate- 
rial differs in some mineralogical characteristics. ‘Tremolite is, 
in general, entirely absent ; quartz becomes a noteworthy con- 
stituent ; even the pyrite is sometimes of an entirely distinct 
variety, which resists decomposition ; and the pure gray color, 
common to dolomytes, often supplants the white. In spite of 
the difficulty of identifying the relationship of strata folded to- 
gether in complicated confusion, with important members of 
the series entirely carried away at one point by erosion, at an- 
other completely hid from view by overlying bodies of sand and 
clay, there is ample evidence, at least in the neighborhood of 
Great Barrington, and at points both to the east and west of that 
place, of the occurrence of this upper dolomitic series. The 
