98 
GEOLOGY OF THE FOURTH DISTRICT. 
Mineral Contents of the Group. 
In this group, we find a greater amount and variety of crystallized minerals than in all the 
other rocks of the Fourth District. The absence of disturbance, or the influence of hypogene 
or metamorphic masses, leaves the rocks of this district comparatively barren of simple mine¬ 
rals, which, under favorable circumstances, would have been largely segregated, as we find 
them in some of the lower strata, when in the proximity of crystalline rocks. 
From the decomposition of the shale, results sulphate of magnesia, sulphate of alumina, 
and muriate of soda or common salt, which are found in sheltered situations along the banks 
of the Genesee and Niagara rivers. Sulphate of alumina, in beautiful efflorescent forms, is 
produced from the upper part of the shale during the process of calcination. Nodules of 
gypsum, usually replacing some organic body, are of frequent occurrence. The superior 
extremity of the Caryocrinus ornatus is frequently filled with snowy gypsum, sometimes the 
plates remaining, but a little separated ; at other times, they have entirely disappeared. More 
rarely, cavities are found lined with dogtooth spar. In the upper part of the shale, and lower 
part of the limestone, green carbonate of copper is frequently found. Iron pyrites is univer¬ 
sally diffused through the shale, and hastens its decomposition. 
It is in the limestone, however, that we are to look for the minerals so characteristic of the 
group. The abundant cavities or geodes, resulting in part, if not entirely from the decomposi¬ 
tion of fossil bodies, are always lined or filled with some crystallized mineral. The most 
abundant are dogtooth spar, pearl spar, brown spar, selenite, sulphate of strontian or celestine, 
anhydrite, and more rarely fluor spar, crystallized zinc blende and galena. These minerals 
are variously grouped and associated ; the more common forms, however, are found in all 
situations in the rock, while others are confined to a single stratum or two. 
The calcareous or dogtooth spar is usually of a yellowish color, and in small crystals. The 
fluor spar is in small cubic crystals of a delicate straw color. The blende is often associated 
with pearl spar, some portions of the rock having all the cavities lined with the latter mineral, 
to the almost entire exclusion of the dogtooth spar. The latter is more abundant in the darker 
colored limestone, overlying the light grey which abounds in pearl spar, and is frequently 
associated with the latter of a delicate pink color. 
The celestine often occurs in beautiful groups of crystals, penetrating masses of selenite. 
Fine specimens of the latter are frequently obtained where the spars are enclosed in the 
transparent mass. 
Zinc blende is more common in the higher dark-colored portions of the limestone, and above 
the point where the greater quantity of spar and other minerals occur. It is that part of the 
rock which forms the margin of the cliff at Niagara Falls ; and here as well as at Lockport, 
this mineral is common, though the quantity is not great. Galena often occurs associated 
with the blende, and sometimes in a lower situation ; it is seen in small masses, but more 
generally in threads or thin veins, often apparently as if filling fissures. I have obtained spe¬ 
cimens of this kind at Rochester, where, on fracture, the two sides of the stone presented a 
