The Naturalist in Nicaragua

by Thomas Belt

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* (* "The Glacial Period in North America" by Thomas Belt. Published in "Transactions of the Nova Scotian Institute of Natural Science" 1866 page 91.) 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,* (* See ante, Chapter 4.) 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 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 degrees, 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 of land and water, and that "we are face to face with a problem whose solution must be attempted and doubtless completed by the astronomer."* (* I have since discussed this question in the "Quarterly Journal of Science" for October 1874.)

There is another branch of the subject that I cannot so easily leave. It is the answer to the question, What became of the many peculiar tropical American genera of animals and plants, when a great part of the tropics was covered with ice, and the climate of the lower lands much colder than now? For instance, the Heliconii and Morphos are a group of butterflies peculiar to tropical America, containing many distinct genera which, on any theory of descent from a common progenitor, must have originated ages before the glacial period. How is it that such peculiarly tropical groups were not exterminated by the cold of the glacial period, or if able to stand the cold, that they did not spread into temperate regions on the retreat of the ice? I believe the answer is, that there was much extermination during the glacial period, that many species and some genera, as, for instance, the American horse, did not survive it, and that some of the great gaps that now exist in natural history were then made; but that a refuge was found for many species, on lands now below the ocean, that were uncovered by the lowering of the sea caused by the immense quantity of water that was locked up in frozen masses on the land.

Mr. Alfred Tylor considers that the ice cap of the glacial period was the cause of a great reduction of the level of the sea, amounting to at least 600 feet.* (* "Geological Magazine" volume 9 page 392.) But if we admit that the ice existed in both hemispheres at the same time, we shall have to speculate on a lowering of the level of the sea to at least 1000 feet. We have many facts tending to prove that during the extreme extent of the glacial period the land stood much higher relatively to the sea than it now does. Professor Hartt believes that during the time of the drift, Brazil stood at a much higher level than at present,* (* "Geology and Physical Geography of Brazil" by Ch. Fred. Hartt page 573.) and we can, on the supposition of a general lowering of the sea all over the world, account for the distribution of animal life over islands now separated by shallow seas. Thus Mr. Bland, in a paper read before the American Philosophical Society, on "The Geology and Physical Geography of the West Indies, with reference to the distribution of Mollusca," states his opinion that Porto Rico, the Virgins, the Anguilla group, Cuba, the Bahamas, and Hayti, once formed continuous dry land that obtained its land molluscs from Central America and Mexico. The land molluscs of the islands to the south, on the contrary, from Barbuda and St. Kitt's down to Trinidad, are of two types, one Venezuelan, the other Guianian; the western side of the supposed continuous land, namely, Trinidad, Tobago, Grenada, the Grenadines, St. Vincent, and St. Lucia, belonging to the first type; the eastern side, from Barbados to Antigua, to the second.* (* Quoted in "At Last" by Charles Kingsley page 305.)

Commenting on Mr. Bland's valuable communication, Mr. Kingsley justly says: "If this be so, a glance at the map will show the vast destruction of tropic land during almost the very latest geological epoch; and show, too, how little, in the present imperfect state of our knowledge, we ought to dare any speculations as to the absence of man, as well as of other creatures, on those great lands destroyed. For, to supply the dry land which Mr. Bland's theory needs, we shall have to conceive a junction, reaching over at least five degrees of latitude, between the north of British Guiana and Barbados; and may freely indulge in the dream that the waters of the Orinoco, when they ran over the lowlands of Trinidad, passed east of Tobago, then northward between Barbados and St. Lucia, afterwards turning westward between the latter island and Martinique, and that the mighty estuary—for a great part at least of that line—formed the original barrier which kept the land shells of Venezuela apart from those of Guiana."* (* Loc cit page 306.)

A very similar theory has been propounded by Mr. Wallace to account for the distribution of the faunas of the Malay Archipelago, in his admirable work on the natural history of that region.* (* "The Malay Archipelago" volume 1 page 11.) Java, Sumatra, and Borneo are separated from each other, and from the continent of Asia, by a shallow sea less than six hundred feet in depth, and must at one time have been connected by continuous land to allow of the elephant and tapir of Sumatra and Borneo, the rhinoceros of Sumatra and Java, and the wild cattle of Borneo and Java, to spread from the continent to these now sea-surrounded lands, as none of these large animals could have passed over the arms of the sea that now separate them. The smaller mammals, the birds, and insects, all illustrate this view, almost all the genera found in any of the islands occurring also on the Asiatic continent, and the species being often identical. On the other hand, the fauna of islands to the eastward are more closely connected with Australia, and must at one time have been joined to it by nearly continuous land. Honeysuckers and lories take the place of the woodpeckers, barbets, trogons, and fruit thrushes of the western islands, and the many mammals belonging to Asiatic genera are no more seen.

Mr. Wallace ascribes the present isolation of the islands, and their separation from the adjoining continents, to the submergence of the channels between them caused by the abstraction of matter thrown out by the numerous volcanoes. Looking, however, at the fact that at the time when these islands were probably connected with the continents of Asia on the one side and Australasia on the other, namely, at the close of the pliocene period, England was connected with the continent; Malta, as shown by its fossil elephants, with Africa; the West Indies with Yucatan and Venezuela; it seems to me more probable that the cause was not a local one, but a general lowering of the waters of the ocean all over the world to at least one thousand feet, produced by the prodigious quantity of water locked up in the frozen masses that covered a great part of both hemispheres.