1887. | NEW YORK ACADEMY OF SCIENCES. 3k 
Division-planes of the Dolomyte. 
Whatever the exact system of crumpling may have been, in 
that far distant time when the edges of our continent were 
shrinking along the ocean-border, and the mountains of western 
Berkshire were swelling up into vast folds, there can be no 
question but that the heavy pressure must have resulted in 
a notable condensation of all the materials thus squeezed up to- 
gether. We might expect to find at such points, where the folds 
were sharpest and the pressure most intense, a cementation or 
soldering of materials into solid masses ; and this in fact we do 
find at this point in the valley. ‘The schist or gneiss at some 
places is compacted, in spite of its original slaty structure, into 
a dense material which becomes a good building-stone. ‘To the 
same intense pressure, soldering together the sharp folds, the 
dolomyte masses which occupy the Housatonic valley are in- 
debted for their consolidation and remarkable compactness. 
During the folding, the main pressure was probably borne by 
the upper and lower thick masses of stiff gneiss and quartzyte. 
Of these the upper, of which a remnant now makes up the Hast 
Mountain, then formed a great fold over the position of the 
present valley. After the main folding had been accomplished, 
reaction ensued, bringing down vertically upon the dolomyte 
layers the superincumbent weight, with a force equal to that 
required for the elevation. 
By the pressure of the dolomyte, while still in a plastic state, 
up against the crown of this arch, its own particles rearranged 
themselves at right angles to the vertical line of pressure, so as 
to produce a new horizontal bedding. An accompanynig hori- 
zontal thrust of the whole mass of strata-dolomyte, gneiss, and 
quartzyte, probably northward against the mass now represerted 
by Monument Mountain, has produced, within the dolomyte, 
another series of lines of division, vertical and running east and 
west. 
The original material appears to have been a thinly laminated 
or slaty magnesian limestone, probably possessed of little more 
compactness or strength than much of the “ Stockbridge lime- 
stone’? of the layer below. By the intense pressure and the 
heat thereby produced, this earthy magnesian slate was con- 
verted into a crystalline marble. Where compressed in a few 
folds, as at Mt. Peter, the pressure was sufficient to produce a 
compact rock, which, though still slaty, has yielded a very fair 
building material for over a half century to the citizens of 
Great Barrington. 
But on the east side of the Housatonic, the series of five 
sharp and narrow overturned folds, into which the dolomyte 
