Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/32

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water in a siinflar locality. We can readily conceive that carbonate of lime chemically deposited may get very intimately mingled with mnd, 80 that the whole may be more or less calcareous. If in such situations there be any alternation of causes, such, for instance, as a river or riyers at one time bearing down bicarbonate of lime in clear streams, and at others, when in flood, transporting fine detritus, there might be difierent deposits, forming alternate layers of limestone and clay. Occasionally, from the tendency to separate, we should expect nodules of limestone to be formed in layers, particles of the fine mud in all cases being much intermingled with the calcareous portions.

Ordinary oceanic or sea currents, as distinguished from those pro- duced by tidal action, though chiefly due to sur&ce causes, such as the effect of prevalent winds, sometimes produce sets of water in ^ven directions that may considerably affect the distribution of detrital matter brought within their influence. In the Gulf Stream we possess a marked instance of a powerful current caused by the Atlantic water, driven westward and pent up in the Gulf of Mexico, escaping to the northward. Where great rivers, such as the Amazon, discharge them- selves into these moving masses of water, a distribution of the finer detritus borne seaward must be produced in accordance with the general line of movement

It is interesting to consider the modification of deposit which may be effected along such a sea line as that of the eastern coasts of America from Cape Horn to Baffin's Bay. From Terra del Fuego, excepting the Straits of Magellan, to the West Indian Islands, we have an oceanic coast under the action of tides, to which detritus from the in- terior is variously borne by the drainage waters. In the Gulf of Mexico and the adjoining West Indian Seas we have, as regards the distribution of detritus, a tideless sea communicating with the tidal coasts of North America by the strong current of the Gulf Stream. In the Bay of Fundy there is a good example of a great rise and fall of tide, caused by local circumstances producing corresponding effects, the resulting distribution and accumulation of mud, sand and gravel mnigliiig graduaDy with those arising from the different rise and fall obtained on the otiier side of the neck of land connecting Nova Scotia with New Brunswick. The gulf and river of St Lawrence present us with otiier conditions, and Baffin's and Hudson's Bays with others. On this coast, therefore, one extending many thousand miles, we expect a great variety of contemporaneous deposits, both as regards organic contents and mineral structure, due to a variety of conditions, the whole highly instructive when we examine and connect in great groups varieties of rocks formed at equal geological times.

Having thus sketched some of the modes in which mineral matter is

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