VI.—Laccolites

Extract

In the Report on the Geology of the Henry Mountains, Rocky Mountain Region, Mr. G. K. Gilbert, of the U.S. Geographical and Geological Survey, points out that many of the intrusions of eruptive rocks now exposed had a deep-seated origin; the molten rock having filled vacancies in the rocks, and never coming to the surface until they were exposed by denudation or by faults. To quote our author, “The lava … instead of rising through all the beds of the earth's crust, stopped at a lower horizon, insinuated itself between two strata, and opened for itself a chamber by lifting all the superior beds. For these masses of eruptive rocks, Gilbert proposes the name laccolite (Gr. lakkos cistern, and lithos stone). In the Cos. Wexford and Wioklow some of the protrusions of eruptive rocks are entitled to this name, the rocks having congealed in cisterns below the surface of the earth; there are, however, some marked differences between them and the laccolites of the Henry Mountains. The latter were intruded into nearly horizontal strata, the laccolites only consist of one kind of rock, while the adjoining rocks seem to have been very little altered. But the Wexford and Wicklow laccolites, on the other hand, were intruded into highly disturbed strata, they are made up of a variety of rocks, and always the aquo-igneous action due to their intrusion—‘ baked ’ or altered, a greater or less thickness of rocks about them.