48 The American Geologist. jan. i89o 
No. 3), where from a distance we may study the manner in 
which the islands have been encroached upon by the ocean. 
The line of the surface of the eastern extremity of the group, 
and the tops of the outlying rocks, evidently formed portions 
of the same original slope, through which the sea made open- 
ings or passages which in time caved in from the tojD and 
formed distinct islands and outlying rocks still projecting 
above the surface of the water. 
In this manner the islands have been broken through at 
the weakest points, thus dividing the Anacapas into three 
distinct islands ; and the channels between the principal 
islands of the chain have doubtless been formed in the same 
manner. 
The middle and eastern Anacapas are composed entirely of 
volcanic rocks with the exception of a superficial deposit of 
water-worn pebbles and fragments of quartzose and metamor- 
phic rocks extending diagonally across the top of the middle 
island near its southeasterly corner, and from which the 
aborigines selected material for the manufacture of their 
weapons. Many of these fragments show evidence of having 
been broken and flaked off" by the hand of man. 
At a point on the south side of the island there is a deposit 
of limestone a few feet below the surface. Another deposit of 
drift occurs at the easterly end of the west island ; a vein of 
milky chalcedonic quartz of about 10 inches in thickness is 
seen some 35 or 20 feet below the drift near the arched passage 
or "Natural Bri-dge" near the east end of the west island. 
Westerly from the latter point this stratified uplifted rock 
rises abruptly until it attains a hight of almost 1,000 feet, 
with a dip of 45 degrees toward thfe north. This portion of 
the island was not explored to any extent, except as we rowed 
or sailed along the channel side where the bluffs rise perpen- 
dicularly from the water. At the beach near the "Natural 
Bridge" the prevailing rock is an amygdaloid and vesicular 
basalt containing spherules of zeolites. 
The basaltic base of the middle island is composed largely 
of black vesicular basalt, containing spheroid and amygdaloid 
pebbles of chalcedony, which weather out and roll down the 
banks in the form of marbles. Some of these are solid, others 
are hollow and lined with drusy quartz. 
