CAMBRIAN. 51 



TrimiclcHS, Calyuiene, PJiacops, Illamis, and Honialonotus 

 among the Trilobites ; Crania, Leptcsna, and Strophomcna, 

 among the Brachiopods ; MjircJusonia and Eiiomphaliis 

 among the Gasteropods. Corals also appear.' 



The Cambrian rocks constitute a great series of grits and 

 slates, with subordinate beds of limestone ; and they contain 

 many interbedded and intrusive igneous rocks. 



Their total maximum thickness is estimated at upwards of 

 35,000 feet in Wales, and at about 20,000 feet in the Lake 

 District, but in the latter area the base is not seen. It must 

 also be remembered that, owing to the disturbances and 

 changes the beds have undergone, such estimates are very 

 hypothetical. 



The Cambrian rocks have, on the whole, been formed 

 in shallow seas, but there are some evidences of tolerably 

 deep water. The nearest land-areas lay probably to the 

 north. We have some records of the weather in the " fossil- 

 ized sunshine and showers," preserved to us in the sun- 

 cracks and rain-pittings ; and Dr. Hicks is of opinion that 

 in the earlier stages of the Cambrian period the climate was 

 probably very cold, gradually becoming milder, until in 

 time warm currents or seas of moderately high temperature 

 prevailed, as indicated by the growth of corals.- The 

 Mollusca and the Trilobites indicate marine conditions, and 

 the sandy and muddy sediments indicate the nearness of 

 land. 



The later stages of this period were characterized by 

 much volcanic activity, and Sir Andrew Ramsay has remarked 

 that our terrestrial scenery, so far as we can restore it, 

 then consisted of groups of volcanic islands. Some of the 

 volcanoes may have rivalled Etna in height, but no traces of 

 any terrestrial flora have been found. In North Wales the 

 ashy series of beds thickens more and more in an easterly 

 direction, suggesting that prevalent westerly winds had a ten- 

 dency during eruptions to blow the volcanic dust and lapilli 

 eastwards.^ 



1 Hicks, Q. J. xxxi. 190. 



2 G. Mag. 1876, pp. 160, 250. 



•* Phys. Geol. and Geogr. Gt. Britain, ed. 5. pp. S0-S2. 



