The Transportation of Boulders 203 
that have been greatly denuded, grain gold is only sparingly 
disseminated throughout the drifts of the valleys, whilst in 
Australia every auriferous quartz vein has been the source 
of an alluvial deposit of grain gold, produced by the denuda- 
tion and sorting action of running water. When the denud- 
ing agent was water, the rocks were worn away, and the 
heavier gold left behind at the bottom of the alluvial deposits ; 
but when the denuding agent was glacier ice the stony masses 
and their metallic contents were carried away, or mingled 
together in the unassorted moraines. 
That the transportation of boulders in Nicaragua was due 
to glaciers, and not to floating icebergs, may be argued on 
zoological grounds. The transported boulders, near Ocotal, 
are about three thousand feet above the sea, those near 
Libertad about two thousand feet. The low pass between 
the Atlantic and the Pacific oceans, through the valley of the 
San Juan and the Lake of Nicaragua, is less than two hundred 
feet above the sea, and to allow for the flotation of icebergs 
at the lower of the two places named, a channel of more than 
eighteen hundred feet in depth would have connected the 
two oceans. ‘This supposition is negatived by the fact that 
the mollusca on the two coasts, separated by the narrow 
Isthmus of Darien, are almost entirely distinct, whilst we 
know that since the glacial period there has been little change 
in the molluscan fauna, nearly, if not all, the shells found in 
glacial deposits still existing in neighbouring seas. In the 
Caribbean province, which includes the Gulf of Mexico, the 
West Indian Islands, and the eastern coast of South America 
as far as Rio de Janeiro, the number of marine shells is 
estimated by Professor C. B. Adams at not less than 1500 
species. From the Panamic province, which, on the western 
coast of America, extends from the Gulf of California to 
Payta in Peru, there has been catalogued 1341 distinct 
species of marine molluscs. Out of this immense number of 
species, less than fifty occur on both sides of the narrow 
Isthmus of Darien. So remarkably distinct are the two 
marine faunas, that most zoologists consider that there has 
been no communication in the tropics between the two seas 
since the close of the miocene period, whilst the connection 
that is supposed to have existed at that remote epoch, and 
tScee ante, p. 31. 
