70 BULLETIN OF THE 
entirely of fragments of corals, of mollusks, and other invertebrates, 
cemented together into a moderately compact oólitio limestone, which 
when discolored, as it often is, and turned gray, might readily be mis- 
taken for sandstone. This coral rock is covered by just such a thin, 
ringing coating of limestone as characterizes the modern reef rock of 
other localities. On nearly all the islands there are a number of sandy 
beaches made up of decomposed fragments of corals and other inverte- 
brates, and cemented together at or beyond high-water mark into the 
modern reef rock I have described. The coral is mainly made up of 
fragments of Pocillopora, which is found covering more or less extensive 
patches off these coral sand beaches, but which, as is well known, never 
forms true coral reefs in the Panamie district. The only true coral reef 
belonging to this district is that of Clipperton Island (if we can trust the 
Admiralty charts), situated about 700 miles to the southwest of Aca- 
pulco. But neither at Cocos Island, nor at the Galapagos, nor anywhere 
in the Panamic district, do we find true coral reefs, — nothing but iso- 
lated patches of reef-building coral. The absence of coral reefs in this 
district has of course already been noted by other naturalists, who have 
been struck by this feature in an equatorial region. Dana has ascribed 
it to the lower temperature of the water due to the action of the Hum- 
boldt Current coming from the south, pouring into the Bay of Panama, 
and then flowing westward with the colder northerly current running 
along the west coast of Mexico and Central America. From the investi- 
gations made this year by the ‘ Albatross,” I am more inclined to as- 
sume that the true cause of the absence of coral reefs on the west coast 
of Central America is due to the immense amount of silt which is 
brought down the hill and mountain sides every rainy season, and which 
simply covers the floor of the ocean to a very considerable distance from 
the land, the terrigenous deposits being found by us even on the line 
from the Galapagos to Acapulco, at the most distant point from the 
shore to the side or extremities of our line. The mud in Panama Bay 
to the hundred fathom line is something extraordinary, and its influence 
on the growth of coral reefs is undoubtedly greatly increased from the 
large amount of decomposed vegetable matter which is mixed with the 
terrigenous deposits. 
Tue ORIGIN or toe Fauna AND FLORA or THE GALAPAGOS ISLANDS. 
In an article on “ The Origin of the Galapagos Islands,” in the Amer- 
ican Naturalist, (March and April, 1891,) Dr. Baur has expressed views 
on the origin of the fauna and flora of the Galapagos entirely at variance 
