570 H. W. FAIRBANKS 
probably of later date, as they are clearly not surface flows. The 
ash reaches in places a thickness of many hundreds of feet, and 
in the hills south of San Luis Obispo passes downward into an 
agglomerate of bowlders of perlitic glass, many of them of 
large size. 
The surface flow of rhyolite on Old Creek is very interesting. 
It is only a few feet in thickness and imbedded in a great mass 
of tuffs of the same character. The sheet terminates at one 
edge in flattened nodular masses from one-half inch to eight 
inches in diameter. Some of these are entirely free, others more 
or less connected in the plane of flow. Superficially many of 
these nodules appear like concretions or large spherulites, but 
their internal character is entirely different. The flowage lines 
pass through them regularly without regard to the shape of the 
surface, and the interior appears to have shrunken away from 
the center, from which radiating cracks either empty or filled 
with chalcedony spread toward the outside. These cracks break 
across the banding with but little disturbance of the latter. 
They do not appear to be real spherulites and the field relations 
suggest that their peculiar character may be due to sudden cool- 
ing under water. 
The augite teschenite and olivine diabase form two generally 
distinct variations of one magma. The former rock contains 
varying proportions of analcite and its alteration products, 
augite and a basic feldspar. The typical examples are rather 
light colored rocks, but with the appearance of olivine and a 
decrease of feldspar and analcite the rock becomes very dark 
and basic looking. Some of the olivine diabase contains so 
little feldspar that it might with almost as much propriety be 
termed a peridotite. These rocks are among the most interesting 
of any yet discovered in the Coast Ranges, and as far as is yet 
known the augite-teschenite is confined to Santa Barbaraand San 
Luis Obispo counties. It has been described in former papers.’ 
Not only is it petrographically interesting, but remarkable for its 
structural relations to the Miocene in which it is intruded. 
* Bull. Depart. of Geol., Univ. of Cal., Vol. 1, p. 273; zd2d., Vol. II, p. 19. 
