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between Depilto and the hill three miles south-west of Ocotal.
Gravel with boulders of trap
Gravel with boulders of gneiss
and quartz rock.
Quartz rock and gneiss.
will not be surprised that in a hurried visit of only a few hours I should not have discovered any. Glacial scratches are seldom preserved on rock surfaces exposed to the action of the elements. Even in Nova Scotia, where scratches and grooves are met with wherever the rock surface has been recently laid bare, I do not remember having ever seen any on natural exposures. It is only where protected by a covering of clay or gravel from the action of the elements, that they have been preserved through the ages that have passed since the glacial epoch, and as I did not see any rock surfaces near Depilto that had been recently bared, it is not surprising that, notwithstanding the other proofs of glacial action, I should not have seen any ice scratches or grooves.
I could no longer withstand the evidence that had been gradually accumulating of the presence of large glaciers in Central America during the glacial period, and these, once admitted, afforded me a solution of many phenomena that had before been inexplicable. The immense ridges of boulder clay between San Rafael and Yales, the long hog-backed hills near Tablason, the great transported boulders two leagues beyond Libertad on the Juigalpa road, and the scarcity of alluvial gold in the valleys of Santo Domingo, could all be easily explained on the supposition that the ice of the glacial period was not confined to extra-tropical lands, but in Central America covered all the higher ranges, and descended in great glaciers to at least as low as the line of country now standing at two thousand feet above the sea.
In my description of the mines of Santo Domingo I have only briefly alluded to the scarcity of alluvial gold in the valleys. It may be correlated with a similar
scarcity in the glaciated valleys of Nova Scotia and North Wales, in the neighbourhood of auriferous quartz veins, and is probably due to the same cause. Glacier ice scoops out all the contents of the valleys, and in deepening them does not sort the materials like running water or the action of the waves upon the sea coast. I have in another place * shown that in Nova Scotia, in the neighbourhood of rich auriferous quartz veins 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 denudation and sorting action of running water. When the denuding 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 Glacial Period in North America,” by Thos. Belt. Published in Trans. Nova Scotian Institute of Natural Science, 1886, P. 93.
+ See ante, p. 35.
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 to account for the distribution of corals, whilst advocated by Professor Duncan and other eminent men, is disputed by others equally eminent. No zoologist of note believes that there has been a submergence of the land lying between the Pacific and the Atlantic since the pliocene period, and icebergs could not have floated without such submergence, so that, in the cases I have mentioned, the boulders, if ice-borne, have been carried by glaciers and not by floating ice.
Whilst I thus found evidence of the ice of the glacial period reaching, in the northern hemisphere, to within the tropics; in the southern hemisphere Professor Hartt has found glacial drift extending from Patagonia, all through Brazil to Pernambuco, and Agassiz has even announced the discovery of glacial moraines up to the equator. I have myself seen, near Pernambuco, and in the province of Maranham, in Brazil, a great drift deposit that I believe to be of glacial origin; and I think it highly probable that the evidence that is accumulating will force geologists to the conclusion that the ice of the glacial period was not only more extensive than has been generally supposed, but that it existed at the same time in the northern and southern hemispheres, leaving, at least, on the American continent, only the lower lands of the tropics free from the icy covering
I shall not enter upon the question of the cause of the cold of the glacial period. It is probably closely connected with the cause of an exactly opposite state of things, the heat of the miocene period, when the beech, the hazel, and the plane, lived and flourished in Spitzbergen, as far north as latitude 78°, and, according to Heer, firs and poplars reached to the North Pole, if there was then land there for them to grow upon. I consider that the great extension of the ice in the glacial period supports the conclusion of Professor Heer, founded on the northern extension of the miocene flora, that these enormous changes of climate cannot be explained by any rearrangement of the relative positions