« EelmineJätka »
Ch. XIV.] THE LAURENTIAN FORMATION. 259
at the mouth of one of the old shafts. Our guide told us that the lode was two feet wide. Both it and the containing rock was very hard, and the miners had also water to contend against. I do not think from what I saw that the mine could be made to pay on a large scale, though next the surface small remunerative deposits of ore had been found. In depth the hardness of the rocks would make the sinking of shafts and driving of levels, the “dead work” of the miners, very costly. 42 We started on our return down the valley at three o'clock, and took particular note of the succession of the rocks, as I had become much interested in finding these quartz and gneissoid beds, which I had no doubt were the same Laurentian rocks that I had seen in Canada and Brazil, the very backbone of the continent, ribbing America from Patagonia to the Canadas—the fundamental gneiss which is covered in other parts of Central America that I had visited, by strata of much more recent origin. Going down the valley of the Depilto the massive beds of quartz and gneiss are soon Succeeded by overlying, highly inclined, and contorted schists, and as far as where the road from Ocotal to Totagalpa crosses the river, the exposures of bed rock were invariably these contorted schists, with many small veins of quartz running between the laminae of the rock. On the banks of the river, from about a mile below Depilto, unstratified beds of gravel are exposed in numerous natural sections. These beds deepen as the river is descended, until at Ocotal they reach a thickness of between two and three hundred feet, and the undulating plain on which Ocotal is built is seen in sections
near the river to be composed entirely of them. These unstratified deposits consist hostly of quartz sand with numerous angular and Subangular blocks of quartz and talcose schist. Many of the boulders are very large, and in some parts great numbers have been accumulated in the bed of the river by the washing away of the smaller stones and sand. Some of these huge boulders were fifteen feet across, the largest of them lying in the bed of the river two miles below Depilto. Most of them were of the Depilto quartz rock and gneiss, and I saw many in the unstratified gravel near Ocotal fully eight miles from their parent rock. Near Ocotal this unstratified formation is nearly level, excepting where worn into deep gulches by the existing streams. The river has cut through to a depth of over two hundred feet, and there are long precipices of it on both sides, similar to those near streams in the north of England that cut through thick beds of boulder clay. The evidences of glacial action between Depilto and Ocotal were, with one exception, as clear as in any Welsh or Highland valley. There were the same rounded and smoothed masses of rock, the same morainelike accumulations of unstratified sand and gravel, the same transported boulders that could be traced to their parent rocks several miles distant. The single exception was, I am convinced, one of observation and not one of fact, viz., I saw no glacial Scratches on the rocks; but geologists know how rare these are on natural exposures in districts that have certainly been glaciated, and 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 Ame"rica 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, and probably much lower. 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
Ch. XIV.] THE LAND HAS NOT SUBSIDED. 263
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 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 mol
* “The Glacial Period in North America,” by Thos. Belt. Published in Trans. Nova Scotian Institute of Natural Science, 1866, p.93. + See ante, p. 35.