230 
GEOLOGY OF THE FOURTH DISTRICT. 
Diagonal lamination .—This structure, which indicates the mechanical process of deposition, 
is of frequent occurrence. In many of the flagstones these lines of deposition are marked by 
thin laminae of shale, and again the layer thins entirely out. The process is readily under¬ 
stood to be that where the sand is carried on and spread over the surface, sloping off towards 
one side farthest from its origin. The next deposition covers this sloping side necessarily in 
the same manner, producing the oblique lines which are often seen in these flagstones. 
Ripple marks are abundant in the sandy shale, or where the shale becomes interstratified 
with sandstone, but it is often difficult or impossible to obtain good specimens. They are 
irregular or unequal generally, but in some instances are well preserved. Many of them have 
the appearance of having been produced by a “chopped sea,” or that where a current opposed 
the direction of the wind. The same effect is often visible on sandy beaches where the 
tide has ebbed ; the surface being broken into short, interrupted and irregular ripples, pro¬ 
duced either by the tidal current opposing the wind, or some other similar cause, always 
readily understood. That such circumstances operated at the time of this deposition, and 
that the sea was alternately shallow and again deeper, is proved at every step among these 
strata. The deposits of dark and green argillaceous shales bear no evidence of ripples or 
diagonal lamination, and from their homogeneous nature were probably deposited in deep 
water, while all the sandy shales and alternations of shale and sandstone bear evidence of 
a shallow sea. It is true that the homogeneous mud would not retain ripple marks or 
lines of deposition, but from its nature and the attendant circumstances we infer that it was 
deposited in deeper water and under a more quiet condition than the sandy strata. 
Casts of Shrinkage cracks. — These have been described under the Medina sandstone, and 
in the Portage group we again meet with them, though not so prominent or so large as in 
the former. 
These marks are upon the under surface of an argillaceous sandstone which rests directly 
upon a soft black shale. The shale became cracked by exposure above water, and the sandy 
matter subsequently deposited filled the fissures. 
Concretions. — This group throughout presents a great variety of concretionary forms. 
The strata, however, are all uniform, and rarely give any appearance of concretionary struc¬ 
ture in themselves. The concretions are more or less calcareous in the different parts of the 
group and in different shales. The more perfectly spherical with seams of crystalline matter 
are found in the black shale, while the forms varying from this to the very flat or lenticular 
ones are found in the shales varying from blackish to greenish black and green. The black 
varieties of shale are always more or less slaty, while the green varieties are not slaty, but 
present the marly or indurated mud-like characters, and instead of separating into laminae, 
crumble into irregular fragments. 
The more spherical forms are due to a higher degree of crystallization which results from a 
larger proportion of carbonate of lime, while the flattened forms are less crystalline, and con¬ 
sequently less spherical from the larger amount of argillaceous matter. 
