234 
GEOLOGY OF THE FOURTH DISTRICT. 
marks and mud-cracks both indicate a shallow sea, and these surfaces often offer equal 
evidences of the same. It frequently happens also that in the same layer presenting these 
casts of flowing mud there are diagonal lines of lamination, indicating the power and action 
of currents upon the transported matter, being another fact in proof of the shallowness of 
the sea. 
Many other illustrations might be given presenting the same general character, and in 
none of them is there any appearance like the ordinary concretionary structure. From nume¬ 
rous examinations of this kind, I have been led to the conclusion that these are due to the 
cause here assigned. Having seen no description of any thing of this kind, I have been under 
the necessity of attributing them to some cause; and if this be not the true one, I shall be 
glad to embrace any other view which will more readily and naturally explain them. 
Casts of Mud-furrows and Strice. 
I have applied this term to certain appearances upon the under side of the strata of sand¬ 
stone, or flagstones, which are numerous and extensive in the Fourth District, as well as 
elsewhere in this group; having from my own observation detected them in other parts of 
New-York, in Pennsylvania, Ohio, and even to some extent in Indiana. 
These casts are elevated lines or ridges upon the surface of the stone, varying from the size 
of the usual scratches upon the present surface of the strata to the diameter of half an inch, 
and even one, two and three inches, and in one case I have seen a specimen six inches in 
diameter. 
The only assignable cause for these ridges is the action of a current flowing over the sur¬ 
face of the strata, sometimes transporting sand and at other times coarser materials, which 
furrowed the surface upon which the subsequent deposits were made. They are, in all cases, 
preserved upon the under surfaces of sandstone or shaly sandstone layers, which rest upon 
soft shale, so that the furrows or scratches must have been made in this mud. They are not 
all confined to one position, but appear at different depths in the group ; showing that the 
cause, be it what it may, operated through a long period, and in a pretty uniform manner. 
The ridges are never curved or bent on one side; and though two systems are sometimes 
observed crossing each other, they are still as well defined and their course as unbroken as 
in the glacial or alluvial scratches upon the surface of the present rocky strata. 
From the frequent occurrence of these, and their continuation through a great thickness of 
strata, we can hardly suppose the furrows to have been made upon hard surfaces; and if we 
suppose the mud in which they were made to have been soft, it seems almost impossible to 
conceive how they should be preserved. Still the numerous similar facts in other rocks 
prove that even the most delicate markings are preserved, under even more unfavorable cir¬ 
cumstances. The tracks of birds and reptiles in the new red sandstone, with the impression 
of rain drops, is equally difficult to comprehend, were it not demonstrated beyond all question. 
Again it has been shown that in the Medina sandstone* the delicate wave lines and the minute 
Pages 52 and 54 of this volume. 
