. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Ch. xxvn.] LLANDEILO FLAGS. 565 occurs under the form of trappean tuff (volcanic ashes of De la Beche), as in the crest of Snowdon, the peculiar species which dis- tinguish it from the Llandeilo beds are still observable. The forma- tion generally appears to be of shallow-water origin, and in that respect is contrasted with the group next to be described. Professor Rarnsay estimates the thickness of the Bala Beds, including the con- temporaneous volcanic rocks, strat
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Ch. xxvn.] LLANDEILO FLAGS. 565 occurs under the form of trappean tuff (volcanic ashes of De la Beche), as in the crest of Snowdon, the peculiar species which dis- tinguish it from the Llandeilo beds are still observable. The forma- tion generally appears to be of shallow-water origin, and in that respect is contrasted with the group next to be described. Professor Rarnsay estimates the thickness of the Bala Beds, including the con- temporaneous volcanic rocks, stratified and unstratified, as being from 10,000 to 12,000 feet in thickness. Llandeilo Flags.—The Lower Silurian strata were originally divided by Sir R. Murchison into the upper group already described, under the name of Caradoc Sandstone, and a lower one, called, from a town in Caermarthenshire, the Llandeilo flags. The last-mentioned strata consist of dark-colored micaceous flags, frequently calcareous, with a great thickness of shales, generally black, below them. The same beds are also seen at Builth in Radnorshire, and here they are inter- stratified with volcanic matter. A still lower part of the Llandeilo rocks consists of a black car- bonaceous slate of great thickness, frequently containing sulphate of alumina, and sometimes, as in Dumfriesshire, beds of anthracite. It has been conjectured that this carbonaceous matter may be due in great measure to large quantities of imbedded animal remains, for the number of Graptolites included in these slates was certainly very great. I collected these same bodies in great numbers in Sweden and Norway in 1835-6, both in the higher and lower graptolitic shales of the Silurian system; and was informed by Dr. Beck of Copen- Fig. 649. (Old plate, fig. 599, p. 442.) Fig. 650. Didymograpsus (Graptolites) Murchisonii, Beck. Llandeilo flags. Wales. Diplograpsus pristis, Hi singer, sp. Shropshire; Wales ; Sweden, &c. Llandeilo flags. F
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
