blendic, in the metamorphic process, easily soften and bend."
Those
authorities assuming the igneous origin of the hornblende rock, as
dikes, dissent naturally from this explanation, and Julien,
acknowledging the evident original plasticity of these hornblende
masses, has in a rather generalized way attributed it to condensation
by pressure in the crustal shortening, to softening by heated waters
and mineralizers, and to the internal lamellar structure of its
components, permitting slipping, twisting, etc. To me it seems that
here this admirable observer fails in conclusiveness.
THE IGNEOUS ORIGIN OF THE HORNBLENDE ROCKS
Dr.
Julien rejects the reference of the hornblende rock to the metamorphism
of aluminous clays or shales holding a high percentage of iron oxides,
lime, and magnesia, because an analysis of the New York hornblende rock
does not agree with the analyses of hornblende schists certainly
derived from such sediments. He also dismisses the suggestion that the
hornblende rock represents changed layers of volcanic ash simply on the
same ground, that the analysis of such tuffs does not correspond with
the table of composition assigned to the Manhattan Island rock by Dr.
Jouet, of the Columbia School of Mines.
He
further objects to the hornblende having originated in the change of
magnesian calcareous and iron constituents in a dolomite or limestone.
In this process the limestone became amphibolised or changed in
part to amphibole, of which hornblende represents a highly ferriferous
variety. After such a change the removal of the residual calcite or
dolomite by solution would leave a hornblende or amphibole rock. An
extensive series of comparisons convinces him that the hornblende
rock of Manhattan Island did not originate in this way.
By exclusion our author is driven to the third hypothesis, viz., metamorphism of basic igneous intrusions, or the change
