chemical nature was yet uncertain or altogether unknown, and his name
was appended to one of the species which he defined. He studied
also, and with success, the interesting modifications called
pseudomorphism--the mode of association of minerals, as well as their
magnetic properties. The attributes of a practical mineralogist aided
him greatly in the culture of a branch of geology to which Delesse has
rendered eminent services, in the recognition of rocks of igneous origin
and of others allied to them. He studied in the field, as well as by
investigations in the laboratory, for fifteen years, with an intelligent
and indefatigable perseverance, and, aided by the results of hundreds of
analyses, eruptive masses of the most varied kind, the knowledge derived
from which threw light upon the principles of science, from granites
and syenites to melaphyres and basalts. After thirty years of study
and progress, other _savants_, without differing from him, progressed
further in the intimate knowledge of rocks; but the historian of
science will not forget that Delesse was the precursor of this order
of research. His studies of metamorphism will long do him honor. The
mineralogical modifications which the eruptive rocks have undergone in
the mass are the permanent witnesses which attracted all his attention.
The chemical comparison of the metamorphic with the normal rock pointed
out distinctly the nature of the substances acquired or lost. One of the
principal results of these analyses has been to lessen the importance
attributed until then to heat alone, and to show in more than one case
the intervention of thermal sources and of other emanations from below,
to which the eruptive rocks have simply opened up tracks.

It is not only upon subjects relating to the history of rocks that
Delesse has touched. Witness his work on the infiltration of water, as
well as his volume relating to the materials of construction, published
on the occasion of the Exhibition of 1855. The nature of the deposits