128 
GEOLOGY OF THE FOURTH DISTRICT. 
This rock holds the place of the porous or vermicular limestone of the Third District, which 
in many places closely resembles a vesicular lava. In the Fourth District these pores are 
rarely larger than flax seeds, though sometimes several of them communicate with each other, 
and not unfrequently they are filled with gypsum. 
The following illustration is from a specimen where the weathering has slightly enlarged 
the cavities, and presents the common appearance of the larger pores. From this size, they 
diminish till they become microscopic. 
51 . 
In addition to the kind of cavities just described, we often meet with small linear ones, 
which, in some parts of the shaly and marly limestones, are abundant. These are probably 
produced, as in the Water-lime group, by the removal of crystals. For an illustration of these 
forms, see the description of the Water-lime group in the following pages. 
Fourth or Upper Deposit of the Group. 
There is rarely any well defined line of demarcation between the shales and shaly lime¬ 
stones of the last deposit, and this division. Still, however, it is often of considerable thick¬ 
ness, and as a whole quite different from the rocks below. It is called by Mr. Yanuxem the 
Magnesian deposit , from the striated or columnar surfaces like those described under the 
Niagara group, page 95. These form a distinguishing feature, which does not appear in the 
lower rocks of the group. 
This division consists usually of thin-bedded impure limestones, drab or ash-colored, and 
sometimes presenting a bluish tint on first exposure. The composition is that of a silico- 
argillaceous limestone, and is the rock from which nearly all the hydraulic cement in the dis¬ 
trict is obtained. From this circumstance, it has been distinguished as the Water lime , or 
Hydraulic limestone, in the Annual Reports of the district. Finding, however, that the rock 
