BIGSBY PALEOZOIC ROCKS OF NEW YORK. 253 



called secular oscillations, have produced, we must remember, vast 

 changes in the nature of the sediments bj withdrawing the sea- 

 bottoms from certain influences and exposing them to others. They 

 are often repeated on the same spot, and create new features in 

 land and sea, — ^new islands, continents, straits, and broad oceans. 

 Shutting up old communications, they may open new ones, and 

 introduce currents laden with other faunae and florae. Hence, it is 

 evident, arise a multitude of zoological complications: migration 

 begins in one place, is stopped at another ; a community of living 

 beings perishes, to be replaced by a new group. Similar observa- 

 tions may be made on sudden uplifts, sometimes endowed with 

 tremendous and far-extending energy ; leaving behind them, at the 

 same time, more or less of metamorphism. 



The characteristics of pelagic, estuarine, and fluviatile deposits, 

 so important in palaeozoic investigations, have been well explained 

 by Edward Forbes and Constant Prevost, and must be familiar to 

 my readers. We need not dwell on the effects which oceanic waters, 

 their vegetation, and suspended matters exert upon the production 

 and perfection of animal life. These effects result from the 

 character of the mineral contents of the sea, its depth, distance 

 from land, and its temperature. The saline ingredients of the sea 

 must have always been nearly the same as at present, as well as 

 the degree of dilution. The presence of iron, of free acids, and of 

 many other foreign substances is commonly fatal to animal life. 

 When the first of these abounds in fossiliferous strata, it has usually 

 been introduced after the death of the resident animals. Depth of 

 sea and distance from land are important considerations as regards 

 life, as has been admirably shown in the zoological zones and par- 

 allels estabhshed by Edward Eorbes. 



In reference to temperature, we find that differences in latitude 

 had not the same effect in palaeozoic times that they have had since. 

 Everything points to the prevalence of a uniform and rather high 

 temperature during these primal times and long afterwards. Their 

 palaeozoic species are widely disseminated. An Australian or an 

 Arctic species may be found in a Chinese, British, or American rock ; 

 and this not as an exceptional fact. Every small section of the 

 thermometric scale, embracing perhaps only a few degrees of heat, 

 has its own assemblage of life as regards the present seas. M. 

 Deshayes points out that the number of species increases at the 

 present day as we approach the equator, — ^there being only 10 or 12 

 in latitude 80°, and, by progressive increase, more than 900 in the 

 seas of Guinea ; such is the life -multiplying power of heat. We 

 must always remember that marine life requires for its well-being 

 a resting-place, security, food, and a proper medium for the per- 

 formance of physiological acts. 



§ 2. Paloeozoic Sediments; their nature. — The complete development 

 of the Silurian system in Wales and the border- counties*, its volcanic 



* In the frequent references throughout these pages to the palaeozoic area of 

 Wales and the adjacent English counties, the single word " Wales " only will be 



employed. 



t2 



