ANNELID TRAILS. 1$ 
ridgelets, 2 mm. long, raised at an angle of 40 deg. to the mid 
line, which is marked by a very slender ridge. In this 10 ridgelets 
cover a centimetre ; the double row is 4r5 mm. wide. 
The morsel of shale bearing these trails, though much less than 
half an inch (9'5 mm.) in thickness, is an accumulation of eight 
distinct strata, whose edges, appearing on a shelving portion of its 
upper surface, are there manifested by sinuous lines crossing it, and 
defining the successively diminishing extents of accretion. Over 
the whole of the outcrops, and over nearly the whole of the upper 
surface of the slab, can be traced the path of the worm, but not 
with uniform ease. Until it reaches the edge of the newest layer 
it is but an impression obscure in its details, and uninstructive were 
it not a guide to the interpretation of trails in a similar condition 
elsewhere. On the upper surface the terminal third or thereabout 
of the trail is in a like state of imperfection. The intermediate and 
characteristic portion was evidently made over mud which was at the 
moment more ductile under the worm's motile organs. So was it 
also in the case of the shorter trail, which ensued immediately on the 
emergence of a worm from mud capable of rising in proof of the 
fact of a passage across it, for it may be well to point out that 
the present trails are not casts made by the lower surface of a 
superincumbent bed, but disturbances of mud actually traversed. 
The laminated shale, which in thus recording events in ancient 
history enormously antedates the clay tablets inscribed by Babylonian 
hands, is more communicative than its artificial relatives, inasmuch 
as it informs us of some at least of the conditions under which 
itself was brought into existence and its story written. The data it 
hands down point to a muddy shore, on bay, estuary, or wherever 
else tides ebb and flow under a calm and clear sky. A mud flat, so 
called, is far from being on a dead level throughout; it has its 
swellings and its sinkings, and in its hollows, as in rock basins, 
water is apt to be left by every retreating tide. On one of these 
ancient shores neap tides brought each its burden of finely levigated 
silt, and deposited it seaward of that left by the preceding flow. At 
intervals between tides the water in the depressions of the surface 
dried up more or less in the heat of the day, and here and there the 
surface hardened or remained soft as circumstances ruled ; worms 
issued from beneath the watery parts and trailed across the sediment 
