Walcott.J CORRELATION. 40? 



The occurrence of Scoliihus linearis and the general aspect of the fossils also show 

 that these rocks must be very nearly, if not exactly, in the same geological horizon 

 with the Upper Primal sandstones and slates of Pennsylvania. 1 



The Lower Paleozoic rocks of Newfoundland were correlated with 

 those of Canada by Sir W. E. Logan upon the evidence of the fossils 

 and their stratigraphic position. The fossils were identified by Mr. 

 Billings, and the stratigraphic geology was studied by Messrs. Richard- 

 son, Murray, and Howley. 



Dana. — Prof. J. D. Dana availed himself of the correlations made by 

 the New York, Canadian, and other surveys in the preparation of the 

 earlier editions of his Manual of Geology. The Manual is a masterly 

 compilation of the available data upon broad principles of correlation, 

 and as it went at once into the hands of all American geologists and 

 students its influence has been widespread. The principles of correla- 

 tion mentioned are outlined in the following paragraphs : 



But the question may arise whether a geological age is not, after all, strongly 

 marked off in the rocks. Rocks are but the moving sands or the accumulations of 

 dead relics of the age they represent, and are local phenomena, as already explained. 

 Each continent has its special history as regards rock-making, and it is only through 

 the fossils in the rocks that the special histories are combined into a general system. 

 Movements have in all ages disturbed one hemisphere without affecting the other, 

 causing breaks in the succession of rocks in one continent or part of a continent that 

 have no representatives in another. 



When an age can be proved, through careful study, to have been closed by a catas- 

 trophe or a transition which was universal in its effects, the event is accepted as a 

 grand and striking one in geological history. But the proof should be obtained 

 before the universality is assumed. Hence the conclusion : 



Fourthly. The grander subdivisions or ages in geological history based on organic 

 progress should be laid down independently of the rocks. They are universal ideas 

 for the globe. The rocks are to be divided off as nearly as practicable in accordance 

 with them. 



Each continent, under these ages, then becomes a special study, and its history has 

 its periods and epochs which may or may not correspond in their limits with those of 

 the other continents. Every transition in the strata, as from limestone to sandstone, 

 clay beds or conglomerate, or from either one to the other, and especially where there 

 is also a striking change in the organic remains, indicates a transition in the era from 

 one set of circumstances to another; it may be a change from one level to another in 

 •the continents, a submergence or emergence or some other kind of catastrophe. All 

 such transitions mark great events in the history of the continent, and thus divide 

 the era into periods, and periods into epochs, and epochs, it may be, into subepochs. 

 Hence — 



Fifthly. Through the ages each continent had its special history ; and the periods 

 and epochs in that history are indicated by changes or transitions in the rock forma- 

 tions and their fossils. 



It is greatly to the assistance of research that some of the revolutions of the globe 

 have probably been nearly or quite universal. The one preceding the Mammalian 

 age appears to be an example ; although, even with regard to this, further. investiga- 

 tion is required before its actual universality can be regarded as established. But 

 the periods aud epochs of America and Europe are not in general the same in their 



'Paleozoic fossils. Containing descriptions and figures of new or little known species of organic 

 remains from the Silurian roctks. Vol. 1, 1860-1865, p. 2. Montreal, 1865. First printed in 1861 in a 

 bulletin iasued by the Geological Survey of Canada. 



