170 The American Geolo^rist. September, looi. 
are found between those pyroxene rocks rich in feldspar, the 
latter mineral fomiing over one-half of the mass, and those 
without a trace of that constituent. A common type is one con- 
taining- less than one-quarter of its bulk of plagioclase, the rest 
being formed of hypersthene. 
The mode of occurrence of the non-feldspathic rocks indi- 
cates that they are younger than both noryte and gabbro. The 
peridotyte area near Mount Hope church, for examole, is plain- 
ly an intrusive mass in the older noryte rock, and their more 
recent age is even more apparent in the case of the dikes. 
It has been shown that the serpentine undoubtedly origin- 
ated from the alteration of the peridotytes and pyroxenytes so 
closely associated with it. The gradual passage of the latter 
into serpentine is well exhibited in many of the less altered 
specimens, where all stages of the change can be traced. These 
ultra-basic types must therefore have occupied at one time a 
much larger area than at present and covered the district now 
composed of serpentine. 
A somewhat different alteration product of the pyroxenytes, 
and one which has perhaps resulted more frequently from their 
transformation, is an amphibole rock. This is composed of a 
fibrous hornblende, sonxetimes green in color ( smaragdite), 
sometimes gray. The different individuals occasionally have 
a core of unaltered pyroxene, which indicates that the mass was 
originally composed of this mineral. These secondary amphi- 
bole rocks are abundant throughout the area, but appear 
to be especially common near the serpentine border where, as 
already stated, the pyroxenytes often occur. 
CHEMICAL RELATIONS OF THE ROCKS OF THE SERIES. 
The chemical relations of the rocks of the district are shown 
in the following table in which the analyses are arranged in the 
order of diminishing acidity. 
I. II. III. IV. V. VI. VII. VIII. 
Si02 73.69 66.68 58.57 55.16 44.0+ 48.02 53.21 40.06 
Ti02 0.50 1.41 0.64 2.24 0.23 0.26 
Zr02 0.09 0.02 0.10 none trace 
AI0O3 12.89 14.93 16.10 17.51 20.01 20.01 1.94 1.37 
V0O3 0.02 0.04 0.05 0.02 0.03 
Cr203 , none trace none 0.03 0.20 0.2O 
FC2O3 1.02 1.58 2.89 2.62 4.22 1.13 1.44 
