138 The American Geologist. September, i904 
The relief is high (Fig. B, PI. VII). The only alteration pro- 
ducts noted are serpentine and occasionally hornblende. 
Hypersthene occurs in irregular patches associated with 
olivine and magnetite. Pleochroism is distinct, pale greenish 
to pink. The extinction is always parallel to the pinacoidal 
cleavages. 
Hornblende is very common in this rock, both in the nu- 
cleus and in the matrix which separates the spheroids. The 
black minerals are well shown in the figure of the hand spec- 
imen. 
The periphery merges into the surrounding rockmass. 
There is no abrupt change in the preceding variety. The 
change from the radial zone to the nucleus is also gradual. 
Chemical Properties. 
One partial analysis was made of the obicular gabbro of 
the third variety. Samples were taken from all parts of the 
fresh surface. The sample was made to represent, as nearly 
as possible, the entire composition of the rock. With this 
analysis are given, for comparison, the analysis of a spheroid 
given by professor Lawson*, and a partial analysis of an 
orbicular noryte from Plumas Co., Cal, described by ]\Ir. H. 
W. Turner.! These are seen belov\' : 
] 
II. 
III. 
SiO= 
43 
.28 
40 
.08 
45-92 
AhOs 
\ 
22 
.86 
FeO 
Fe.Oa 
2,2,- 
69 
( " 
.96 
CaO 
12 
.02 
II 
•41 
9.61 
MgO 
II 
■77 
12 
.40 
13.85 
Na.O 
I 
.26 
•71 
K.0 
.38 
.10 
I. Orbicular gabbro, Dehesa. Horowitz, analyst. 
II. Spheroid of orbicular gabbro, Dehesa. Howson, anal^'st. 
III. Orbicular noryte, Plumas Co. Steiger, analyst. 
A comparison of I and II does not show any great change 
in the chemical contents between the spheroid and the matrix. 
That the matrix is higher in SiOo may be due to the fact that 
the hornblende predominates, while in the spheroid the feld- 
spar predominates. The calcium and magnesium are practi- 
• L,oc. cit.. p. 394. 
t 17th Ann. Rept., U. S. Geol. Surv., p. 64-2. 
