518 HISTORICAL GEOLOGY. 



cover an area of not less than 550 miles and have a thickness of about 8000 

 feet ; the rocks are felsytes, andesytic and other lavas, and volcanic tufa. 



3. The Bala (Sedgwick, 1838), or Caradoc group (Murchison, 1839): con- 

 sisting of shales, flags, and sandstones, with some limestone. — The Caradoc 

 rocks in Shropshire are about 4000 feet thick, while the Bala, in the Bala 

 district, Merionethshire, have a thickness of only 1100 to 1200 feet, and the 

 chief limestone stratum is only 20 or 30 feet thick near the middle. The 

 Coniston limestone, the equivalent of the Bala, has a thickness of 200 feet. 

 The Upper Coniston beds are Upper Silurian. 



In Caernarvonshire, northwestern Wales, great eruptions took place 

 in this period, makmg eruptive accumulations 6000 to 8000 feet thick. The 

 rocks are porphyries, felsytes, andesytes, besides diabases. Ireland, also, 

 had its eruptions. 



4. The Lower Llandovery group. The beds have a thickness in South 

 Wales of 600 to 1000 feet, but they are absent from North Wales. They 

 consist of shales, flags, sandstones, and conglomerates. The Upper Llando- 

 very is closely related to the Lower in rocks and fossils. The two were 

 separated, and the former made the base of the Upper Silurian, by Sedgwick 

 in 1853, who called them the May Hill sandstones. This arrangement is 

 adopted by Geikie. 



The thickness of the Lower Silurian rocks of Wales has been estimated 

 at 25,000 feet. But over a fourth of this is owing to volcanic contributions, 

 which, as they are of an extraordinary source, should be set aside in compar- 

 ing the thickness of the sedimentary beds of different regions with reference 

 to elapsed time. In the south of Scotland the thickness is over 16,000 feet. 



It is not possible to make out a precise parallelism between the British 

 and American strata. Approximately the Arenig group represents the Cal- 

 ciferous ; the Llandeilo flags, the Chazy; the Bala and Caradoc, the Trenton; 

 and the Lower Llandovery, the Utica and Hudson beds. 



The Lower Silurian and Cambrian formations of Norway, Sweden, Russia, 

 and Bohemia, which rest upon Archaean rocks, have but little thickness — 

 1000 to 2000 feet ; and, adding what denudation may have carried away, 

 4000 or 5000 feet would be a large estimate for the original amount. 



In northern and northwestern France, or Normandy and Brittany, Lower 

 Silurian rocks occur in a much upturned condition. The gr^s Armoricain is 

 a sandstone, according to Barrois, of the age of the Chazy and Trenton 

 limestones. Below it, and also above it, are shales or slates, and those above 

 may represent the remainder of the Lower Silurian. They are found, also, of 

 similar character in the Astarias, northern Spain, and in the Pyrenees. 



In Bohemia, the Lower Silurian of the basin of the Prague is the Stage 

 D, or 2d Fauna, of Barrande. It consists of shales, with some quartzyte and 

 conglomerate below, and has a thickness of about 3000 feet. 



In southern Sweden (Scania), the beds are mostly shales, many of the 

 beds Graptolitic, with some limestone ; and are divided into a Lower, Middle, 



