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Fig. 4.--Section from Mickle Aw Fell to Murton Pike, Lake District.
(Prof. R. Harkness.)
d. Upper Greenstones, Porphyries, and Ashes.
a. Skiddaw Slates.
e. Bala or Coniston Limestone. b. Greenstones, Ashes, and Porphyries.
g. Carboniferous Rocks. * Upper Old Red Sandstone.
c. Fossiliferous Flaggy Shales.
Bowder stone is a peculiar metamorphosed slate. Green slates are best developed on the south-eastern slopes, and occupy a long range of mountains parallel to the Skiddaw slates, and in those highly picturesque and romantic valleys wherein the lakes of Ulleswater, Haweswater, Thirlmere, and Wastwater spread their beautiful waters. (Phillips.)
The occurrence of the Borrowdale series in the Isle of Man has been pointed out by Professor Harkness and Dr. Nicholson.
In consequence of the superior hardness of the rock, and its frequent association with igneous products, the Green slate mountains assume bolder forms, present more lofty and rugged peaks, and more inaccessible precipices, than the softer slates of Skiddaw. (Phillips.)
1 This woodcut is borrowed from Mackintosh's Scenery of England and Wales, p. 238.
CARADOC, OR BALA BEDS. UPPER CARADOC. (J. Phillips, 1842.) The Caradoc Sandstone was first noticed by Murchison in 1833, and in 1834 he described the strata under the name of Horderley and May Hill Sandstone. Subsequently, in the Silurian System (1839), they were called Caradoc Sandstone from the circumstance of their being typically developed in the neighbourhood of Caer Caradoc. (Ramsay.)
The term Bala Beds was given by Sedgwick.
The typical Bala or Caradoc beds lie in the Bala district, between Dinas-Mowddwy, Bettws-gwerful-goch, and Bettwsy-Coed. They consist of black and blue slates, and grey and brown arenaceous beds; the Bala limestone, generally very impure, lying about the middle, and averaging from 20 to 30 feet in thickness. Between the limestone and the lower traps of the Arenigs and Llyn Conwy, two, and sometimes three thin and imperfect beds of volcanic ashes represent the whole of the vast volcanic accumulations of Moel Hebog, Snowdon, and Carnedd Llewelyn. The middle part of the Bala beds, including the limestone, is most fossiliferous, the black slates below, and the slaty and sandy interstratifications above, being comparatively barren. (Ramsay.)
The thickness of the series is estimated at from 10,000 to 12,000 feet. Murchison states that the limestones and sandstones in North Wales do not present a thickness of more than 1,050 feet, while those of Shropshire attain about 4,000 feet.
The beds are found in Anglesea, but the best localities are Caradoc, Horderley (shelly sandstones), Norbury, Bala, and Snowdon. The Meifod beds may also be included. Northwest of Bala, the Rhiwlas limestone---a grey limestone, 30 to 40 feet thick, contains many fossils.
North of Moel-Siabod (according to Professor Ramsay) the Bala beds assume a markedly different character from that which they possess between Dinas-Mowddwy and
d. Volcan c Ashes, sometimes calcareous and fossiliferous=Bala Limestone.
a Fossiliferous Grits overlying slaty beds. b. Greenstone (intrusive).
e. Columnar Felspathic Rock.
c. Felspathic Porphyry.
Dolwyddelan, for they contain a much greater number of interbedded felstones and volcanic ashes, which range northward to Conway, and from thence south-west along the higher Caernarvonshire mountains. Carnedd Llewelyn, CarneddDafydd, Y-Glyder-fawr, Snowdon, and Moel-Hebog are the chief mountains in this, the wildest and grandest part of North Wales. And these, like the ranges of Cader Idris, the Arans, and Moelwyn, consist in a great degree of volcanic products. These volcanic rocks belong to two sections of the (Cambrian) period, for the felstone porphyries and felspathic ashes, and perhaps even the intrusive greenstones of Merionethshire, were formed during the deposition of the Llandeilo strata, while the same species of thickbedded traps and ashes on Snowdon and the surrounding mountains are high in the Bala or Caradoc series. In both cases they form the highest mountain ranges in Wales, not from upheavals caused by the intrusion of igneous masses in special areas, but simply from the circumstance that long after their formation, while lying deep below thousands of feet of newer strata, the whole of the rocks of the area bave been disturbed ; and the hard igneous masses now rise so high because they have better withstood degradation than the slaty rocks with which they are interbedded. The ranges formed of the lower porphyries, &c., of Cader Idris, Aran Mowddwy, Arenig, and Moelwyn, lie midway up in the strata of the great Merionethshire anticlinal, while the peaks of the still higher range of Moel-Hebog, Snowdon, and Carnedd Llewelyn actually lie in the middle of a basin. The whole form but minor parts of an old mountain system, of which Wales is only a fragment. (Ramsay.)