512 HISTORICAL PALAEONTOLOGY. 



Lakes Superior and Huron. They are about 18,000 feet in 

 thickness, and consist of quartzites (altered sandstones), sili- 

 ceous slates, conglomerates, and limestones. The conglome- 

 rates sometimes contain pebbles derived from the subjacent 

 Laurentian rocks. No fossils have hitherto been found in 

 any part of the Huronian series, and its exact age is therefore 

 doubtful. Not improbably it may correspond with the Lower 

 Cambrian rocks of other regions, but it may represent an 

 independent formation to be intercalated in point of time 

 between the Laurentian and Cambrian groups. 



CAMBRIAN PERIOD. 



ROCKS OF THE PERIOD. The exact limits of the Cambrian 

 Rocks are as yet not well denned, different authorities taking 

 different views as to the strata which should be considered 

 under this head. The name " Cambrian " is derived from the 

 fact that these strata are the lowest rocks visible in North 

 Wales and its borders (Cambria). The Cambrian rocks are 

 generally divided into a Lower and Upper division, and they 

 are well developed in various parts of Europe and America. 

 The following gives a general idea of the nature, distribution, 

 and mineral characters of the Cambrian rocks : 



I. Cambrian Rocks of Britain. The Lower Cambrian rocks 

 of Britain are best seen in the Longmynd Hills in Shropshire, 

 and consist of about 25,000 feet of variously-coloured sand- 

 stones, grits, and shales, often ripple-marked, and exhibiting 

 rain-prints, but with very few fossils. These are succeeded by 

 a great series of micaceous flagstones, slates, and shales, which 

 vary in thickness from 6000 to 2000 feet, and are, in part 

 at any rate, of Upper Cambrian age. They are known as the 

 Lingula flags, from the occurrence in them of a Brachiopod 

 belonging to the genus Lingula. In North Wales the Lower 

 Cambrian strata are often highly metamorphosed, and the cele- 

 brated Welsh roofing-slates are also derived from this division. 

 Cambrian rocks occur in other parts of Britain, and the follow- 

 ing table exhibits their leading members : 



1. Lower Cambrian : 



a. Longmynd beds (25,000 feet). 



b. Llanberis slates (3000 feet). 



c. Harlech grits (6000 feet). 



d. Oldhamia slates of Ireland. 



2. Upper Cambrian: 



e. Lingula Flags of Wales (about 6000 feet). 



/ Tremadoc Slates of North Wales (2000 feet). 



g. Skiddaw Slates of the north of England (7000 feet). 



