E. S. Dana—Garnets from the Trap Rocks of New Haven. 215 
Art. XXX.— Mineralogical Notes. No. V. On the occurrence 
of Garnets with Sh oe of New Haven, Connecticut; by 
by Epwarp 8. 
GARNETS have been recently discovered in connection with 
the New Haven trap rocks, at two distinct localities ; and their 
method of occurrence presents some points of considerable 
interest. 
These “trap rocks,” as they are commonly called, belong 
to the system of dikes of igneous rocks which characterize the 
Mesozoic sandstone areas of the Atlantic border. Mr. G. W. 
awes has given these rocks a thorough chemical examina- 
tion,* and has shown that they have all essentially the same 
composition, being for the most “pnt true dolerites, but Lg 
ing also the hydrous, chloritic variety called diabase. I tak 
the liberty. of quoting here Mr. Hawes’s analysis of the teu 
from West Rock, which, as he states, is “the typical rock of 
this region.’ Specific oravity 3°08. 
ive OR Lillis cos ee eae eee 51°78 
Bape eptipe nee Rear gee 14°20 
a sesquioxide bck eiu ph be ee aus ees 3°59 
Tron protoxide 425 
Manganese protoxide Janie wha Uigbes aan ey can 0°44 
Ailtie ooo oe a a ee 10°70 
Magacis eas bee C eed ae Soa owe ome 7°63 
BOGS oo ec ee ee eae 2°14 
POR ie ee ae 0°39 
Phosphoras pontémide 20. ak 0°14 
RIOR es 0°63 
99°89 
writer has made a microscopic examination,t by means 
of thin sections, of this series of rocks, including, among many 
others, specimens from West Rock (see the analysis above) and 
also from the two localities where the e garnets have been found, 
viz: East Rock and Mill Rock. These three points, it should 
be stated, are within four miles of each other, all lying just 
outside of the limits of the city of New Haven. It was found 
that the specimens from the localities named were Hautes 
in mineralogical character, and that they were all quite free 
from any alteration. The mineralogical constituents are as 
follows: A. triclinic ss Jed which Mr. Hawes has shown 
mor fe o be labradorite, pyroxene, and magnetite, also 
more or less chrysolite, and . little apatite in minute acicular 
crysta. 
< This Journal, III, ix, — — 1875. 
; This wisi) Ii, viii, 
