THE CAMBRIAN PERIOD. 91 



(14) "On the Tremadoc Rocks in the Neighborhood of St. 

 David's, South Wales, and their Fossil Contents. " 

 Hicks. ' Quart Journ. Geol. Soc., ' xxix. 39-52 1873. 



In the above list, allusion has necessarily been omitted to num- 

 erous works and memoirs on the Cambrian deposits of Sweden 

 and Norway, Central Europe, Russia, Spain, and various parts 

 of North America, as well as to a number of important papers 

 on the British Cambrian strata by various well-known observers. 

 Among these latter may be mentioned memoirs by Prof. Phillips, 

 and Messrs Salter, Hicks, Belt, Plant, Homfray, Ash, Holl, &c. 



CHAPTER IX. 



THE LOWER SILURIAN PERIOD. 



The great system of deposits to which Sir Roderick Murchi- 

 son applied the name of " Silurian Rocks " reposes directly 

 upon the highest Cambrian beds, apparently without any 

 marked unconformity, though with a considerable change in 

 the nature of the fossils. The name " Silurian " was originally 

 proposed by the eminent geologist just alluded to for a great 

 series of strata lying below the Old Red Sandstone, and occu- 

 pying districts in Wales and its borders which were at one 

 time inhabited by the " Silures, " a tribe of ancient Britons. 

 Deposits of a corresponding age are now known to be largely 

 developed in other parts of England, in Scotland, and in Ire- 

 land, in North America, in Australia, in India, in Bohemia, 

 Saxony, Bavaria, Russia, Sweden and Norway, Spain, and in 

 various other regions of less note. In some regions, as in the 

 neighborhood of St. Petersburg, the Silurian strata are found 

 not only to have preserved their original horizontality, but also 

 to have retained almost unaltered their primitive soft and inco- 

 herent nature. In other regions, as in Scandinavia and many 

 parts of North America, similar strata, now consolidated into 

 shales, sandstones, and limestones, may be found resting with 

 a very slight inclination on still older sediment. In a great 

 many regions, however, the Silurian deposits are found to have 

 undergone more or less folding, crumpling, and dislocation, 

 accompanied by induration and " cleavage " of the finer and 



