Basic Rocks of Northeastern Maryland. — Leonard. i6i 
together with the small optic angle and well marked trichro- 
ism indicate beyond any doubt that it is hypersthene and not 
enstatite or bronzite. The hypersthene sometimes occurs in 
porphyritic crystals measuring from a quarter to half an inch 
in length (6 to 13 millimeters). The olivine is as a rule not dis- 
tinguishable with the unaided eye except where it appears as 
dull spots on the cleavage surfaces of the pyroxene. It occurs 
in well rounded grains which have commonly been more or 
less altered into serpentine. The manner in which this change 
takes place does not differ from that so often described. The 
fibres of serpentine are arranged at right angles to the cracks 
which fomi a network traversing the olivine in all directions. 
The alteration is accompanied by the separation of considerable 
iron. 
Feldspar is occasionally present in these rocks in small 
amount. As already stated, it had in one specimen been al- 
tered to zoisite. The only other mineral observed was a little 
secondary fibrous hornblende. As a rule the pyroxene found 
in the peridotytes is quite fresh and unchanged. Many of the 
latter rocks which have undergone considerable alteration into 
serpentine still contain scattered through their mass good sized 
crystals of hypersthene or diallage, which show bright cleav- 
age surfaces. The porphyritic structure of the peridotyte 
appears to be the exception rather than the '"ule in this area, 
whereas in the Baltimore region Williams states that all the 
olivine rocks exhibit this structure.* 
Two distinct types of peridotyte appear in northeastern 
Maryland, namely, a diallage-olivine rock or wehrlyte, and a 
hypersthene-olivine rock or harzburgyte (saxonyte). The 
former usually contains a little accessory hyiDersthene and the 
latter may carry diallage in small amount. The two tvpcs 
thus merge into each other and no sharp line of separation can 
be drawn between them. 
The harzburgyte appears to be exceptional in containing 
hypersthene instead of enstatite or bronzite. which arc the 
orthorhombic pyroxenes commonly present in this tvpo. A 
peridotyte of this kind with some hornblende and biotite is de- 
scribed by Cross as occurring in Cottonwood gulch, Custer 
county, Colorado. f The hypersthene was only slightly pleo- 
•Bul. U. S. Geol. Surv., No. 28. p. .50, 188G. and Amer. Geol., July, IS'.K), 
p. 2S. 
tl'roc. Colorado Scientific Society, 1887, p. i;28. 
