Gratacap—Hozoonal Rock of Manhattan Island. 377 
rise and fall as if they had undergone plication, but so strongly 
do they suggest the quartz or granite veins found in the folded 
gneiss layers of the island, that their origin may be connected 
with this very plication, the incipient change in the original 
hornblende schist having been induced at the moment of fold- 
ing, the alteration involving the change of the hornblende to 
serpentine, with the exclusion of lime, which through the 
action of percolating waters finally filled the interstitial spaces 
as calcite. 
An examination of the thin microscope slides, made of sec- 
tions of this ophio-calcite, proved very satisfactorily the change 
of the hornblende into serpentine, threw some light upon the 
process of change,,and lent some support to the view of the 
segregated character of the calcite. 
Groups of hornblende crystals were constantly encountered 
surrounded by the serpentine (figs. 5, 6), while in these, phases 
of alteration were detected and separated. The change of the 
hornblende to serpentine seemed to be frequently preceded by 
a breaking up of the hornblende into fibers, giving the blade 
a striated appearance, while the serpentine granules develop 
along these. In some cases the hornblende blade is replaced 
in patches by serpentine, against which the remaining fibrous 
substance, partially changed, but not deprived of its acicular 
appearance, abuts with a crenulated margin. A second method 
of change seems to consist in a sort of exfoliation, the horn- 
blende stripping up, the alteration entering from all sides 
with no apparent physical change, to the condition of asbestus. 
In this second process the hornblende presents an eroded 
appearance and frequently shows minute cavities, the whole 
aspect being not dissimilar to that of a melting fragment or 
