Grindley— On Certain Tracks in the Manx Slates. 543 
eight or nine inches long, five or six inches wide, and two inches 
deep : in one instance the length was two feet, but only eight or 
nine inches broad ; and from a crack running obliquely across the 
cast near its centre, Mr. Grindley concluded that it was not one 
cast but two, one partly overlying the other. The shape of the 
casts is always the same, independently of their being of different 
sizes; their mineralogical composition, though identical, or very nearly 
so, with the matrix, is generally much softer, and they are often 
washed out of the rocks that lie between high and low water. The 
casts are strongly coloured with iron, and frequently studded with 
small crystals of Blende (the neighbouring rocks contain this mineral 
in great abundance), which, when these crystals happen to fall out, 
present a very curious pitted appearance. The impressions, as a 
rule, occur in pairs, but a single one is often found by itself: when 
however they are in odd numbers, the missing one may be presumed 
to have been covered up by the neighbouring overlying rock, or 
else lost through a break in the continuity of the marked surface. 
They are generally of very unequal size; and the distance be- 
tween the impressions is not always the same, but this is not 
to be expected when the great difference in the size of the pairs 
is considered. In some cases they touch each other ; in others they 
are separated by an interval of several inches. The smallest im- 
pressions are usually closest together ; occasionally the groups are 
clustered in great numbers, at other times scattered over the rocks 
very sparingly. 
The larger axes of the impressions all point in the same direction, 
which is generally speaking, parallel to the line of strike, also to the 
ripple-marks on the rock-surfaces, and consequently more or less to 
the ancient sea-margin. 
Supposing these impressions to be footmarks of some huge Cam- 
brian or Lower Silurian reptile, Mr. Grindley discusses the habits 
which the various situations and positions of the traces indicate, 
and which tend to prove that the creature resorted to the shore in 
search of food. 
This paper, the author states, is rather to draw the attention of 
scientific men to the subject afresh, than to record his own notions 
and observations. He reviews the different opinions expressed by 
geologists as to their real nature ; some have strongly asserted their 
organic origin, while others have as positively denied it. With 
respect to the latter, some have regarded the casts as mere con- 
cretions, others as simply pebbles enclosed in the rock: but the 
casts show no signs of concretion or crystallization, which would 
be inconsistent with the first supposition, and their regularity of 
order would dispose of the second. Others have regarded them as 
double worm-burrows, but there is not the slightest connexion 
between the casts, and they are not deep enough for such to be the 
ease. Among those who admit their organic origin, some suggest 
gigantic mollusks, but this appears to be inadequate to explain the 
fact when their arrangement in pairs, their marked position, and 
their occurrence in rippled slates, an indication of a littoral deposit, 
