50 
GEOLOGY OF THE FOURTH DISTRICT. 
lustration is taken, was from Medina, a few feet above the position of the Lingula, Cytherina, 
See. ; the rock, a variegated quartzose sandstone, showing a slight admixture of the marly 
deposit which immediately succeeds. 
In the terminal grey mass at Medina these markings are very beautifully preserved, in con¬ 
nexion with the Dictuolites. Here the longitudinal direction is north-northeast and south- 
southwest ; and those which the figure represents vary from this but a few degrees, though 
the difference was not accurately determined. 
These facts indicate a shallow sea during the whole time of the deposition of the upper 
part of this rock ; and the diagonal lamination is equally a proof of currents in the ocean dur¬ 
ing the same period. 
In a few instances, I have met with another kind of surface marking, of which I have 
seen no explanation. It is more common in some of the higher groups than in the Medina 
sandstone. The illustration No. 8, is from a specimen of this kind. 
8 . 
The surface of the stone presents undulating ridges, frequently meeting and forming a kind 
of irregular network. An appearance somewhat similar is seen where two plane surfaces 
with wet clay, or other adhesive substance between, are forcibly separated — the clay is thus 
raised in irregular ridges. I have usually referred this appearance to the effect of water eva¬ 
porating from a partially indurated stratum, where the mass below contracts more than the 
thin film upon the surface. From the numerous similar facts, together with more pressing 
and important details, these have been left without sufficient investigation to form a satisfac¬ 
tory conclusion. On the surface of sandstones, alternating with shales in the Portage and 
Chemung groups, this kind of marking is very common, often extending over large spaces. 
