[Assembly 
in stalactitic or mamillary forms^ lining the gecdes or fissures. The 
chalcedonic portions consist, mostly, of organic remains, which appear 
to have been broken and transported before the deposition took place. 
These fossils are of trilobites, Producta, Orthis, &c. This limestone 
contains a stratum of about one foot thick, composed almost entirely of 
the broken shells of Pentamerusj it is very well defined at Rochester^ 
but its position cannot be relied on as will be seen farther on. Above 
the upper green shale, at Rochester, we have the calcareous strata, no- 
ticed on the last page. This rock closely resembles the more calcareous 
portions of the strata above the low^er green shale: it may be described 
as a silico-calcareous rock, extremely hard and tough, containing masses 
and nodules of iron pyrites, which, on decomposing, leave the spaces 
filled by anhydrous gypsum. The decomposition of pyrites, and the 
formation of gypsum in nodules takes place in all the rocks between 
this and the gypseous formation. This rock, when exposed, becomes 
covered with a deposit of an iron rust colour, resulting from the decom- 
position of pyrites, which not only occurs in nodules, but disseminated 
in grains throughout the rock. On this account it is very destructible, 
and wears away by the action of the weather faster than the shale. At 
Rochester, we find the rock just described, and the upper green shale 
entirely distinct from the similar rocks below, and also from the calca- 
reous shales above. On going westward, a few miles, the upper green 
shale disappears entirely, and allows the two calcareous rocks or lime- 
stones to come in contact, and form one rock, with the lower portions 
siliceous and the upper calcareous. After the union of the two rocks, the 
upper portions contain Encrinites, Corallines, Spirifer, Orthis, &c., while 
the lower portions retain the same fossils as when the rocks were distinct. 
The two rocks, after their junction, are only twenty feet thick, whereas 
when separate, the two together were more than thirty feet thick. On 
the Niagara river, the lower portion of this rock is a magnesian lime- 
stone, with pyrites disseminated, causing a rapid decomposition of the 
rock, and the production of a large quantity of sulphate of magnesia. 
The lower green shale, which at Rochester is twenty-three feet thick, 
is only three feet at Medina, and disappears entirely before coming to 
Lockport. At the latter place the shale and grayband are both repre- 
sented by eighteen inches of greenish gray marl. Soon after leaving 
Lockport, the green shale commences again, and on the Niagara river is 
eight feet thick, and probably attains a much greater thickness farther 
west. 
