Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/14



would appear desirable that, from time to time during the progress of the Geological Survey, as sufficient portions of country have been carefully examined, the probable causes which have led to the formation of the rocks composing such districts should be investigated; as, by adopting this course, we may not only be led duly to appreciate the probable condition of these portions of land at different geological times, but also such causes themselves,—those influencing the production of rocks generally. With this view it is proposed to see how far the facts brought to light respecting the geological structure of South Wales and of South Western England may enable us to attain this important object

It may be advisable, before we inquire into these facts, to take a brief view of the effects of the igneous action and of the deposit of mineral matter, chemically or mechanically, from or by means of seas, estuaries, and lakes, as they are at present known to us. By comparing this knowledge with the geological facts observed in the district, we shall see how it enables us satisfactorily to account for them, and how far other reasoning may be necessary.

First, of the igneous products. In various parts of the world ashes and cinders are violently driven out of apertures or vents into the air by expanding gases and vapours, and accumulate in conical heaps around the vents whence they have been expelled. From time to time melted rock is lifted up and overruns or breaks down, or through, the barrier of ashes and cinders, flowing in long lines to various distances, according to circumstances. Occasionally rents are produced in the mass of ashes and cinders, and into them the hot molten rock is forced, rising as any