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 150 it may be doubted, whether any certain proof can be shown that any part of the European region was subjected to great displacement during this period.

It is true that a survey of the porphyries, greenstones, and other igneous rocks, so strangely interlaminated among the clay slates and grauwacke slates of Snowdon, and the middle Cumbrian region, from Black Comb to Ulswater, appears to prove that at certain periods during the formation of these rocks, eruptions of melted rock occurred over a great extent of the oceanic bed; and such we must suppose were accompanied by considerable, if only transient, movements of the solid crust of the globe. Elie de Beaumont has supposed that some of the most considerable displacements of primary strata which are observed in Europe, happened before the completion of the newest of those strata; but it cannot be satisfactorily proved by examples taken from the British Islands. Indeed, every fresh inquiry into the geological dates of particular disturbances of the strata, shows the difficulty of arriving at accurate conclusions on this important subject.

The evidence is sometimes insufficient; in other instances complicated with the effects of convulsions of later date, but similar geographical positions; and however strange it may appear, it is nevertheless true, that the strongest arguments in favour of the convulsions having occurred within particular limits of geological time, have been based on comprehensive views of a whole physical region, rather than on a minute scrutiny and complete survey of the details of the position of the strata, at the line of junction of the displaced and the undisturbed rocks.

After the lapse of most part of the Silurian and before the commencement of the old red period (whatever the interval of time was), great disturbances happened, which uplifted large parts of the bed of the sea, and either raised them above the surface into dry land, or, at least, placed them in such situations that no further deposit of strata was spread upon them at later