Transactions of the Institution of Mining Engineers Volume 3
This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1892 edition. Excerpt: ...Pseudodiadema granukzris. The presence of these remains makes it clear that the rocks, in which they occur, belong to the Cenomanian division of the Cretaceous--somewhere near the position of the Upper Greensand of England. The shales above the limestone are light grey in colour, of a nodular character and contain a considerable amount of lime. The sandstones, under the limestone, are a very dark grey (almost black when wet), micaceous, and much more compact than the shale. The limestone is an ordinary grey, compact stone, not unlike, in appearance, the Carboniferous Limestone of England. Ore Deposits. These occur exclusively in the limestone, and are therefore clearly limited in thickness or depth, at any particular point, by the vertical extent of that rock. This most important fact, from an economic standpoint, does not even yet appear to be recognized, except by those who have had a geological training. The consequence is that it is not uncommon to hear the Bilbao deposits spoken of as "mountains of ore." This erroneous notion arises mainly from the fact that the sides of the mountains have, in places, the same dip as the limestone, or, as the ore, where that has taken the place of the limestone. Under such circumstances it is clear that the whole face ofa mountain might show ore and seem, to the uninitiated, to beta mountainous mass of mineral, whereas, in reality, it would simply be a crust of ore. Such occurrences are not uncommon. The writer has seen them in other parts of the world as well as Spain, and they invariably suggest the same erroneous conclusions to those who have had no training in stratigraphy. In some eases the originally enclosing limestone has been removed from a deposit by denudation, but the horizon...
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