Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/33

20 20 FORMATION OF BOCKS IK SOUTH WALES

known or may be inferred to be now accumulated, we shall proceed to examine the probable manner in which the rocks of the district under consideration have been formed, taking them in their order of geo- logical age.

Silurian Rocks.

As it would be premature to enter upon a general view of the pro- bable mode in which the great mass of rocks, of older geological date than the Old Eed Sandstone epoch, was accumulated in Wales, and in some of the neighbouring English counties, until the whole of that area had been carefoUy examined during the progress of the Geological Survey, the Silurian rocks which appear on the southern portions of Pembrokeshire, ranging thence by Caermarthen, Llandilo, Llangadock, and Llanwrtyd to Builth, with the outstanding and protruded patches of Malvern, Woolhope, May Hill, and Uske, will, for the present, be alone considered, and rather for the purpose of showing the kind of sedimentary deposits which immediately preceded those of the Old Red Sandstone, than as properly entering upon the accumulations of the older palaeozoic rocks themselves.

As most nearly approximating to the district which has chiefly afforded Mr. Murchison the types of the Silurian System, exclusive of the Llandilo flags, it may be convenient first briefly to notice the Silu- rian rocks of Malvern, as they afford us evidence of the accumulations constituting these rocks having been continued beneath the intervening Old Red Sandstone, from the Shropshire districts, with but slight modifi- cation, in thb direction, — ^in fact, showing that similar physical causes had affected the distribution of detrital and calcareous matter over the sea bottom of that old geological period for that distance ; the mud, sand, and gravel banks, as the case might be, having been continuous firom one locality to the other.

Referring to the Memoir* of Professor John Phillips for the needful detail of the Malvern Silurian rocks, the following general section, cor- responding with that given in the Survey Vertical Sections, No. 15, will suffice for this sketch : —

Maivem Silurians,

Feet.

1. White yeUow, and brown sandstones, with partings of shale • • • 100

2. Gray shaly sandstones and thin shales ••.*•• 200

(^Upper Ludlow Socks.)

3. Subcalcareous nodular rock in massive beds 40

(Aytnestjy Xtmestone.)

4. Gray shales, enclosing toward the lower part many bands of calcareous

nodules •«••.•••••• 700

{Lower Ludlow Bock.)


 * InseKed next in order to this Memoir.

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