GEOLOGY. 315 



26. Fossils may be contemporaneous in geological age, without being 

 contemporaneous in time, as commonly understood. 



Geological age is partly determined by fossil evidence. Now, the presence 

 of living beings (subsequently fossil) depends on mineral and other condi- 

 tions, such 'as temperature, depth, currents, etc., which were nowhere the 

 same for large spaces, but were always undergoing changes from plutonic 

 and other causes changes always more or less local and limited, the de- 

 posits being thick or thin in places : so that the universal scheme of palaeo- 

 zoic life was not everywhere worked up to the same point. Here preparations 

 were making for Lower Silurian deposits; there, for the Upper, or Devo- 

 nian, and so on. Thus isochronism was perhaps not common. 



27. The principles of recurrency, succession, increment, and relative 

 abundance of fossil species, are the same in New York, Wales, and else- 

 where, modified by local circumstances. 



28. Recurrency, or reappearance in different strata, is at the same time 

 the measure of viability in the species, and of connection in the groups of 

 strata. It is a kind of living nexus, pointing out that the groups belong 

 to one and the same order of things. It may have been partly caused by 

 migration. 



Recurrency is not so common in New York as in Wales ; in other words, 

 vertical range is longer in Wales. Great depth is an obstacle to the existence 

 or transmission of living creatures. 



29. Everywhere, on the eastern as well as on the western continent, the 

 same fossils, of all orders and kinds, appear in the same succession. A 

 very few Crustacea and a Lingula or Obohts or two, amid a dense matting 

 of fucoids, appear at what now seems to be the dawn of life; then some 

 Gasteropoda, a few Cephalopoda, and a few Brachiopoda in the third group 

 from below (Chazy). But in the fifth group from below (Trenton), multi- 

 tudes of Zoophyta, Bryozoa, Brachiopoda (save Spirifrei), Orthocerata, 

 and Trilobites spring forth; but not a Laniellibranc'hiate. As species, they 

 nearly all perish with the advent of a new deposit; but, as genera, they 

 appear one after another through the successive epochal centres, becoming 

 multiplied in numbers and perfect in form. Then they lessen in numbers, 

 dwindle in size, and finally disappear. 



30. There is a close similarity in New York and Wales in the increment 

 and decrement of Zoophyta, Bryozoa, Echinodermata, Brachiopoda, etc.; 

 that is, these fossils are numerous and few at the same points of the Silurian 

 scale. 



31. The same genera, species, and amount of individuals abound or arc 

 few in the countries just named. Brachiopoda, Crustacea, Orthocerata, are 

 many; Lamellibranchiates few. The extraordinary opulence in fossils of 

 the Rhenish Devonian strata does not obtain in New York. In New York, 

 however, according to our present list, the Lower Silurian stage is the most 

 fossiliferous; in Wales, it is the Upper. Future discoveries may change 

 this condition of things. 



32. A remarkable feature in the uppermost four groups of New York 

 Siluria (the Lower Helderberg) is the substitution in them of limestone for 

 the arenaceous mud of the Welsh Ludlows, their contempoi-aries. It has 

 given them a Wenlock character. But it is to be remembered that the Liul- 

 low and Wenlock groups of Wales are in close fossil connection, 74 out 

 of 311 species of organic remains being common to both, or very nearly 

 one quarter. 



