136 The American Geologist. "ieptembtr, 1904. 
The orbtcnla?' rocks. Three distinct varieties of orbicular 
structure were noted : one shoAving neither concentric nor 
radial structure; another showing concentric, but not radial 
structure; and a third showing both concentric and radial 
structure. 
The first variety was noted in only the most weathered 
rocks. It was found farthest up the hillside, the highest boul- 
der being found about 400 feet from the summit, on the south 
side. It consists of seemingly homogeneous balls which stand 
out on the weathered surface as balls instead of being seen as 
rings, such as the other two varieties show. A fracture of the 
rock merely separates the balls, showing them to be more re- 
sistant than the matrix. The balls are made up of finer crystals 
than the material surrounding them. They are usually about 
30 mm. in diameter, but one was found three inches in diam- 
eter. In places on the surface of the ground these spheroids 
are found to be so plentiful as to give the appearance of "water- 
worn pebbles. In the spheroids of this variety the feldspars 
show evidence of crushing. The olivines occur in rounded 
grains. There is no development noted, either toward con- 
centric or toward radial structure. 
The second variety, of which the largest part of the obi- 
cular boulders is composed, consists of an outer ring of feld- 
spar 3 mm. to 4 mm. in thickness and about 40 mm. in diam- 
eter (Pis. VIII and IX). This surrounds a nucleus which is 
seemingly little different from the outer matrix. In the cen- 
ter is found a sponge-like bunch of feldspars. The feldspars 
in the outer zone have no regular orientation. Concentric 
rings of olivine are seen, but otherwise the zone is composed 
of feldspar entirely. 
The spheroids are usually round, but are sometimes flat- 
tened. In such cases the feldspars show evidence of crushing, 
denoting that movement of the mass took place after consoli- 
dation. There is no^ suggestion of radial structure in this 
variety of spheroid. There seems to be no regularity of orien- 
tation in any of the feldspars. (Fig. B, PI. V). The feldspars 
usually average about 0.4 mm. to 0.5 mm. in diameter. The 
line between this outer feldspar zone, or periphery, and the 
darker minerals of the matrix, is clearly defined as is the line 
between the zone and the nucleus. 
