92 
GEOLOGY OF THE FOURTH DISTRICT. 
nor for the presence of crystals ; for these could only have taken place after the growth and 
formation of the solid structure. 
Both in this and the next succeeding division of the rock, spheroidal cavities are scattered 
in great profusion, and without order. Certain layers, however, yield the finest specimens of 
dogtooth spar, and others of pearl and brown spar; while the selenite and sulphate of stron- 
tian are confined to one or two strata, and are rarely found elsewhere. These facts are of 
interest, and future investigations will probably reach the true cause, whether it may have 
been in certain genera or species of corals, or from other sources that the sulphates resulted. 
The anhydrous gypsum which occurs in this rock, fills cavities in a mass lower than the sele¬ 
nite and sulphate of strontian, and principally below the spar, though often occurring with the 
latter. 
Besides the tendency to spheroidal or concentric concretions, there is sometimes a tendency 
to a thickening of the strata, probably due to the same influences, and which often produces 
apparent undulations in the succeeding ones. An example of this kind is given in the following 
woodcut. It appears in the bank of the canal two miles south of Lockport. The elevation 
is entirely produced by the thickening of the mass, the strata being horizontal below. The 
succeeding strata are seen to thin out as they approach this thicker portion. 
28 . 
Farther south the higher strata seen in this illustration disappear, and a mile beyond this 
point some still higher rise above the level of the canal; these constitute the terminal portion 
of the rock. They are thinly laminated, from the intervention of a black bituminous shaly 
matter. The layers are all curved, the extremities bending downward. The surfaces are 
aeain covered with little knobs or incipient concretions, the convex upper surface of each 
having a corresponding concavity on the under side ; presenting an appearance as if impressed 
with the fingers while in a yielding condition. These appear like what may sometimes be 
seen in a semi-fluid viscid mass when heated, or from which gas is evolved, raising the sur¬ 
face into a great number of small bubbles. Those upon the stone, however, are infinitely more 
