Page:Popular Science Monthly Volume 8.djvu/177

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N selecting a subject for the lecture which, at the request of the council of the British Association, I undertook to give you during its present meeting, I have been guided by the desire to tell you something that would be new to you in regard to matters with which you are already familiar, and to connect this with the results of my own deep-sea researches, in which I might hope that my own local connection with Bristol would lead you to feel somewhat of a personal interest.

In the rocks that border the Avon on either side, the Bristolian has one of the most characteristic examples of limestone that can be anywhere found; and he has only to go as far as the deep gorge of Cheddar, in the Mendip hills, to find limestone cliffs yet more imposing in height than St. Vincent's rocks; or as far as Chepstow, to see, along the Wye to Tintern Abbey, a still more varied and picturesque display of the same great rock-formation. Its material is sometimes distinguished as the mountain limestone, on account of the rugged character it imparts to the districts in which it prevails; while it is now more commonly known as the carboniferous (coal-bearing), because it forms the basins or troughs in which the "coal-measures" lie. Now, if you look at a geological map of England, you will trace this lime-stone as a band lying obliquely northeast and southwest; beginning in Northumberland, passing through Durham and Yorkshire, through Derbyshire (where it forms the romantic scenery about Matlock), then through the midland counties (where, however, it is generally covered up by later formations), and then into Gloucestershire and South Wales, where its relation to the coal-basins is most distinctly marked. Speaking generally, this oblique band divides England into two great areas: one to the northwest, in which the strata that have been brought to the surface, by the crumpling action that has disturbed the crust of the earth during its cooling, are older than the carboniferous lime-stone; the other to the southeast, in which the strata are newer. You have not to go far from Bristol to see examples of both. As you pass down the Avon, you observe a succession of limestone-strata lying obliquely one beneath another; and at last you come to an end of these, and find that the next underlying rock is that Old Red Sand-stone, of which the massive pier on the Somersetshire side of the suspension bridge is built. And Dundry Hill, which is everywhere so conspicuous, is formed at its lower part of Lias, and at its upper part of Oolite, two later formations which were not deposited until after the