5l6 HISTORICAL PALAEONTOLOGY. 



a. The Lower Llandeilo or Arenig group derives its name from 

 the town of Llandeilo, in Wales, where it consists of dark- coloured 

 micaceous flags, with earthy shales and gritty sandstones. 



b. The Upper Llandeilo group consists of a great series of 

 micaceous flags and dark-coloured shales, often with inter- 

 stratified igneous matter. In Scotland the group consists of 

 a great assemblage of shales and grits, the former mostly very 

 dark in colour, often anthracitic, and charged with the remains 

 of Graptolites. 



c. The Bala, Caradoc, or Comston group consists in Wales 

 of slates, grits, and sandstones, with two interstratified lime- 

 stones, the whole attaining a thickness of about 5500 feet. 

 In the north of England this group consists of black flaggy 

 beds, a well-marked limestone with intercalated shales, and 

 black mudstones replete with Graptolites. 



d. The Lower Llandovery group consists of slates and sand- 

 stones, with great beds of conglomerate. 



e. The Upper Llandovery group forms in Britain the base of 

 the Upper Silurians, and rests unconformably upon the Lower 

 Llandovery. It consists of limestones, shales, conglomerates, 

 sandstones, and slates, attaining a total thickness of nearly 

 2000 feet. 



f. The Wenlock group consists of a great mass of shale and 

 flagstone (Wenlock Shale), underlaid and surmounted by lime- 

 stones, the whole attaining a thickness of 3000 feet. 



g. The Ludlow group consists of shales, limestones, and 

 sandstones in Wales, and of grits and shales in the north of 

 England, having a total thickness of from 2000 to 4000 or 

 5000 feet. 



In North America the Silurian series is magnificently devel- 

 oped, and is capable, like that of other parts of the world, of 

 being separated into a Lower and Upper division. The 

 annexed table shows the subdivisions of the Silurian series as 

 developed in the State of New York, and their supposed 

 British equivalents the table being in ascending order : 



Silurian Strata of New York. British equivalents. 



1. Trenton period (comprising the Chazy, Bird's ^ The Lower Silurian 

 eye, Black River, and Trenton limestones. I series (comprising the 



> Llandeilo, Bala, and 



2. Hudson period (comprising the Utica slates Lower Llandovery 

 and Hudson River shales). ' groups). 



\ The lower portion of 



3. Niagara period (comprising the Oneida I the Upper Silurian series 

 conglomerate, Medina sandstone, Clinton group, > (comprising the Upper 

 and Niagara limestone). Llandovery and Wen- 



J lock). 



