152 The American Geologist. September, looi. 
H2O above 105° C... .57 CI midet. 
P2O5 trace Fi undet. 
FeS.J 11{06S)§ 99 98 
CO2 25 Sp.gr 2 980 
HYrEKSTHENE-GABBKO. 
This rock, as already stated, is intimately associated with 
the noryte, into which it everywhere passes through the de- 
crease in the amount of diallage present. It is impossible to 
draw any line between these two types either in their geological 
occurrence or in their mineralogical composition. They occu- 
py the same area and no attempt has been made to separate 
them in plotting the distribution of the different rocks. The 
gabbro is less abundant than the noryte and where it does 
occur it is always rich in hypersthene. It is to be regarded as a 
mineralogical facies of the prevalent noryte. Fine exposures 
are to be seen along the railroad south of Conowingo, where it 
has been blasted to make room for the track. Outcrops also 
occur along Octoraro creek above the mouth of Stone run, and 
in the vicinity of Mount Hope church, while boulders of it are 
abundant at many points throughout the area, notably along the 
Harford turnpike for a mile northeast of Reckord and east of 
Conowingo on the Porter Bridge road. 
The gabbro resembles the noryte in color, ranging from 
light shades in those varieties rich in feldspar to dark colored 
where the pyroxene is the more prominent constituent. As a 
rule they are more compact and do not have the crumbly ap- 
pearance of many of the norytes. In texture they are medium 
to fine-grained. A very coarse-grained facies of the gabbro 
was observed a short distance above the paper mill on Octoraro 
creek, some of the constituents having a length of nearly one 
inch (26 millimeters). 
In the hand specimens the minerals that can be readily dis- 
tinguished are feldspar, hypersthene, and diallage, while under 
the microscope the rock is seen to contain some accessory mag- 
netite and apatite. Olivine was found in only two specimens. 
The . feldspar is bytownite and is in all respects identical 
with that of the norytes already described. It has been more or 
tSulphur calculated as FeSa, but exists as pyrrhotite or other sulphide sol- 
uble in HCl 
§l'erhaps mainly pyrrhotite. 
