nites were mere clay, altered into such forms by
sulphureous waters and subterranean heat; and he pointed out the
different states of shells buried in the strata, distinguishing between,
first, the mere mould or impression; second, the cast or nucleus; and,
thirdly, the remains of the shell itself. He had also the merit of being
the first to point out that some of the fossils had belonged to marine
and some to terrestrial testacea.[49]

_Steno_, 1669.--But the most remarkable work of that period was
published by Steno, a Dane, once professor of anatomy at Padua, and who
afterwards resided many years at the court of the Grand Duke of Tuscany.
His treatise bears the quaint title of "De Solido intra Solidum
naturaltier contento (1669)," by which the author intended to express,
"On Gems, Crystals, and organic Petrifactions inclosed within solid
Rocks." This work attests the priority of the Italian school in
geological research; exemplifying at the same time the powerful
obstacles opposed, in that age, to the general reception of enlarged
views in the science. It was still a favorite dogma, that the fossil
remains of shells and marine creatures were not of animal origin; an
opinion adhered to by many from their extreme reluctance to believe,
that the earth could have been inhabited by living beings before a great
part of the existing mountains were formed. In reference to this
controversy, Steno had dissected a shark recently taken from the
Mediterranean, and had demonstrated that its teeth and bones were
identical with many fossils found in Tuscany. He had also compared the
shells discovered in the Italian strata with living species, pointed out
their resemblance, and traced the various gradations from shells merely
calcined, or which had only lost their animal gluten, to those
petrifactions in which there was a perfect substitution of stony matter.
In his division of mineral masses, he insisted on the secondary origin
of those deposits in which the spoils of animals or fragments of older
rocks were inclosed. He distinguished between marine formations and
those of a fluviatile character, the last containing reeds, grasses,
or the trunks and branches of trees. He argued in favor of the original
horizontality of sedimentary deposits, attributing their present
inclined and vertical position sometimes to the escape of subterranean
vapors heaving the crust of the earth from below upwards, and sometimes
to the falling in of masse