walcott] CORRELATION. 419 



tions. One very great distinction between them, however, is that they 

 have received different dynamic treatment, in the frequency of plutonic 

 disturbance in Wales and its comparative absence in New York, such 

 dynamic treatment involving, it must be kept in mind, both gradual and 

 sudden changes of population. While the strata of these areas are 

 formed of much the same mineral substances, the conditions of these, 

 their order, and quantities are very varied." 1 



He sums up the points of zoological similarity under the following 

 heads : 



(1) The organic remains of both basins belong to the same orders and genera, un- 

 mixed with those of other sedimentary systems, as Permian, Jurassic, etc. 



(2) Vertebrate animals were introduced at nearly the same date. 



(3) The organic remains approximate closely in general facies. 



(4) They affect strata of the same mineral character in both, the majority prefer- 

 ring the calcareous, the others the arenaceous form of deposit. 



(5) The great majority exhibit the same order in their introduction and distribu- 

 tion. This is seen in Orthis, Pentatuerus. Spirifer, and oth^r Brachiopoda, and in 

 Endoceras, Graptolites, Trilobites, etc., the more highly organized being often prom- 

 inent in the early stages. 



(G) The law of divergency into several matrices is the same, or nearly so, in the 

 two basins, the number of instances being fewer in New York. 



(7) The great majority of animals typical or recurrent in one basin are so in the 

 other. 



(8) The great majority of the recurrent fossils in both occupy the same number of 

 e pochs, many or few. 



(9) The great majority observe the same process or law of increment and decre- 

 ment. This takes place in nineteen out of twenty-four orders and genera. 



(10) The two basins have 108 organic forms in common, including most of the gen- 

 era. 



(11) The same orders and genera are rich and poor in species. 



(12) There is the same limited admission of Silurian forms into the Devonian sys- 

 tem in New York and in Europe. 



(13) The plants of both are typical, with one or two exceptions. 



Such are some of the great points of similarity. Now as to dissimilarity. Those 

 which arise out of the mineral character are partly owing to physical disturbances 

 and to a certain amount of metamorphism undergone by the Welsh strata. The 

 paleontological differences are many, but small, often merely individual, and they 

 seldom affect principles. They are due to the varying sea depths and other well 

 known conditions. 



The facts just recorded certainly indicate a close connection in nature and mode 

 of formation between the basins of New York and Wales. They seem to be quasi- 

 equivalents — the same, but other, to use a short, but convenient phrase in common 

 use. 2 



Agassiz. — When commenting upon correlations made between the 

 Faradoxides zone of Newfoundland, Massachusetts, and Scandinavia, 

 Prof. Louis Agassiz said : 



That there may be synchronism of deposit without identity of fossils must be evi- 

 dent if we glance at the present distribution of animals. If at the present epoch 

 the fauna of America and Australia should become fossilized there would not be the 

 slightest resemblance between the representative species of the two continents. The 



1 Quart. Jour. Geol. Soc, London, vol. 15, 1859, p. 292. 2 Op. cit., pp. 292, 293. 



