MUSEUM OF COMPARATIVE ZOOLOGY. 73 
we had an excellent opportunity of studying the natural features of 
these islands, and also as we passed their shores or were dredging 
within a moderate distance. As far as a cursory examination like ours 
could prove anything regarding the nature of the geological structure of 
the islands, our observations fully agree with those of Darwin and of 
Wolf, that this group presents one of the best examples of true volcanic 
islands. 
The majority of the islands are evidently formed around a central 
crater or centre of elevation, They have increased in size and in height 
from successive lava flows. There is nothing to show that the separate 
islands are entirely the result of the disintegration of a larger volcanic 
chain, though of course a certain amount of denudation and submarine 
erosion has undoubtedly taken place, as is readily seen on the slopes of 
the islands and on examination of the soundings between them. Neither 
do we find any indications either of elevation or of subsidence of any part 
of the area of the Galapagos district which would affect their topogra- 
phy, and, as Wolf maintains, we can still less explain their formation 
by a separation in former periods from the South American continent. 
On the contrary, every part of their structure scems to prove that the 
islands have been slowly formed by submarine eruptions at first, and sub- 
sequently by similar accretions at the level of the sea, until finally some 
of the islands have reached an elevation of over 3,000 feet. During 
this process of growth some of the islands have become joined together, 
as for instance Albemarle, which is probably composed of three islands 
originally independent, and also the eastern and western parts of 
Chatham, which were surely once two separate islands, and are now 
connected only by a low isthmus. 
The voleanio activity of some of the islands has continued to com- 
paratively very recent times. I am informed by Mr. Cobos that smoke 
has been seen to issue from Narborough as late as 1836, and it is well 
known that Captain Collet was driven from Tagus Cove by the heat due 
to an eruption on the neighboring Narborough. It is quite probable 
that the age of the Galapagos does not reach beyond the earliest tertiary 
period, and many parts have undoubtedly not been formed before the 
present epoch, so that the time is geologically short during which so 
many animals and plants peculiar to the islands have developed from 
their South American, their Central American, their Mexican, or their 
West Indian ancestors. 
Wolf has already called attention to the fact that the petrographic 
character of the Galapagos volcanoes is different from that of the vol- 
