180 PROF. T. M‘KENNY HUGHES ON SOME TRACKS OF 
concave surface to the fire. So sometimes a thin film of mud 
which has run down a face of rock, or settled from rain-water on a 
smooth surface, curls up like the curled wafers eaten with ices. 
Along the lines thus exposed through the top lamina the next layer 
dries most readily, and so the cracks are continued through layer 
after layer, which are seen projecting in steps along the sides of the 
little abyss now formed. 
This mode of cracking only occurs in certain cases when the 
drying goes on rapidly and the lamine are well-marked off and 
easily parted. 
More commonly the crack cuts through all the layers evenly to a 
depth of from 1 to 2 feet; it gapes some 8 or 10 inches across the 
top, and closes altogether before it reaches the bottom of the tank. 
Such being the character of the deposit and the circumstances of 
its drying, it is clear that if anything begins a groove in the top 
layer, any subsequent cracking would be likely to follow that line ; 
indeed, the trail of some creatures would be as deep as the 
thickness of one lamina. So we see when we are examining 
the pattern of the tracks which traverse the “slurry,” that they 
sometimes follow curious curves, which I have often traced to the 
tracks of large worms, the slight fluting along either edge of the 
track due to the split worm-track enabling one to follow the 
original groove far along an opening which has afterwards attained 
a depth of a foot or more.. 
More rarely we can see how a groove filled with coarser dust or 
sand is well marked off from the layer below, and somewhat, but 
less distinctly, separated from the newer layer which has rushed 
over it. In such cases, when the mass has settled by its weight, 
and shrunk on the evaporation of the water in it, we see that the 
form of the less yielding material which has filled the groove has 
been modified. For, since the portions a a and cc’ (see figure) 
shrink more than the portion 6 b', therefore the ends a and ¢ must 
get squeezed down and the infilled groove present a somewhat 
lenticular section ; and if the groove be quite filled up, the central 
portion 6 will bulge up a little into the overlying bed. 
Diagram Section of Groove. 
sles, SEO Te Se la e 
if v 
a“ pe a a - c 
It will be observed that this answers the objection to the view. 
that Cruziana, for instance, might be a track because it sometimes 
shows a lenticular section, as if it had been a solid body; and also 
shows how in some cases the imprint may appear on the underside 
of an overlying stratum. 
I have frequently seen on a ripple- marked shore that every 
depresssion was filled with black grains of peat or broken blackened 
