FACTS AND OBSERVATIONS. sxxiii 



It is well to add here that the ground occupied by the Upper Silurian strata near Prague, now 

 spoken of, is somewhat irregular, and even rather hilly ; so that a large proportion of the parishes 

 show, each of them, several horizons, belonging to stages E, F, G, and H. M. Barrande has 

 enumerated eighteen parishes in which this occurs. Of these I shall only name Lochkov, Dvoretz, 

 Konieprus, Hlubocep. In the foregoing observations we only deal with the one community residing 

 on the bed E. e. 2. 



UNIVERSALITY. In the spirit of the following definition it would appear that the Silurian 

 system of rocks is universal in extent (that is, it overspreads the whole earth more or less com- 

 pletely, covering up its predecessors), and that its component parts were laid down at a proximate 

 time, and in like manner ceased to be laid down, statements approved by M. Barrande. (Bull. 

 Soc. Geol. de France, n. s. xii. 361.) 



Definition. A formation may be considered to be universal when it occupies large and small 

 areas in very many parts of the earth, often remote from, and even antipodal to each other, when it 

 is always of like stratigraphical relations, is composed of like materials, and contains numerous genera 

 of existences in common, together with some representative and some identical species. 



In support of our applying this definition to the Silurian system, the l Thesaurus ' exhibits the 

 widest possible distribution of its fauna a fauna, it must be remembered, which is pure from 

 admixture with that of any other epoch which might possibly have been progressing at the same 

 time. But we have to except, it must be recollected, the few members of the prior period which 

 have strayed into ours. 



The ' Thesaurus ' contains many examples of the same species of mollusk being in from twenty 

 to twenty-five different countries, countries extensive and far apart, the same creature or creatures 

 marking the route from land to land. This is greatly aided by the power enjoyed by very many 

 mollusks of living on several sediments. In this way the Theca triangularis is found in all the many 

 sediments of the Hudson-River group of America. 



From a Table drawn up under the inspection of Mr. Salter, we find (in 1866) 195 species common 

 to regions very remote from each other, some of them being antipodal a fact which tells the more for- 

 cibly from the tenacity with which the larger part of Silurian life clings to locality as well as to horizon. 



Two hundred and ten species are common to Europe and America. Sixty Silurian genera 

 (truly European) have been brought from South Australia by Mr. Selwyn, the Chief Geological 

 Surveyor of that colony; and Professor M'Coy has met with in that country a Siphonotreta, 

 a Phacops, and eighteen species of G-raptolites absolutely identical with those of North America 

 and of Europe. The Professor strongly expresses his surprise and delight. 



According to M. Barrande, Orthoceras bullatum (Sowerby) is at Melbourne (South Australia), 

 in Ireland, Bohemia, Germany, and Russia ; Conocoryphe depressa is in Wales and Texas (N. A.) . 

 Western Tasmania, the Himalayas, Russia, South and North America, and many other large divisions 

 of the earth afford ample evidence of the general presence of the constituents, zoological and mineral, 

 of this period. 



The following Silurian remains are so widely distributed that they may almost be said to be 

 universal : Calymene Blumenbachii, Orthoceras annulatum, 0. ibex. 0. nummularium, Graptolithus 

 priodon, G. Sagittarius, Leptana depressa, L. sericea, Orthis testudinaria, 0. elegantula, 0. hybrida, 

 O. Wilsoni, Atrypa marginalis, A. reticularis, Pentamerus galeatus, P. Knightii, P. oblongus, Stro- 

 phomena pecten, S. rhomboidalis, Bellerophon bilobatus, Conularia Sowerbyii, Cornulites serpularius, 

 Tentaculites Anglicus, &c. 



The Silurian beds, it must be borne in mind, are usually visible in mere shreds and remainders 

 in the best-worked places. They are apt to consist in any one place of a stage or a part of a stage, 

 the other portion being removed by denudation, or covered up by later deposits for hundreds of 

 square miles ; or they never existed, the locality in the last case having been in a state of emergence 

 during certain periods. 



