here are certain kinds of stones which yield much more
readily to the heat than others. The art of making vitrified forts was
the art of making ramparts of rock through a knowledge of the less
obstinate earths and the more powerful fluxes. I have been informed by
Mr. Patrick Duff of Elgin, that he found, in breaking open a vitrified
fragment detached from an ancient hill-fort, distinct impressions of the
serrated kelp-weed of our shores,--the identical flux which, in its
character as the kelp of commerce, was so extensively used in our
glass-houses only a few years ago.

I was struck, during my explorations at this time, as I had been often
before, by the style of grouping, if I may so speak, which obtains among
the Lower Old Red fossils. In no deposit with which I am acquainted,
however rich in remains, have all its ichthyolites been found lying
together. The collector finds some one or two species very numerous;
some two or three considerably less so, but not unfrequent; some one or
two more, perhaps, exceedingly rare; and a few, though abundant in other
localities, that never occur at all. In the Cromarty beds, for instance,
I never found a Holoptychius, and a Dipterus only once; the Diplopterus
is rare; the Glyptolepis not common; the Cheirolepis and Pterichthys
more so, but not very abundant; the Cheiracanthus and Diplacanthus, on
the other hand, are numerous; and the Osteolepis and Coccosteus more
numerous still. But in other deposits of the same formation, though a
similar style of grouping obtains, the proportions are reversed with
regard to species and genera: the fish rare in one locality abound in
another. In Banniskirk, for instance, the Dipterus is exceedingly
common, while the Osteolepis and Coccosteus are rare, and the
Cheiracanthus and Cheirolepis seem altogether awanting. Again, in the
Morayshire deposits, the Glyptolepis is abundant, and noble specimens of
the Lower Old Red Holoptychius--of which more anon--are to be found in
the neighborhood of Thurso, associated with remains of the Diplopterus,
Coccosteus, Dipterus, and Osteolepis. The fact may be deemed of some
little interest by the geologist, and may serve to inculcate caution, by
showing that it is not always safe to determine regarding the place or
age of subordinate formations from the per centage of certain fossils
which they may be found to contain, or from the fact that they should
want some certain organisms of the system to which they belon