400 THE CAMBRIAN. [bull. 81. 



groups are widely distinct, and their value in determining the comparative age and 

 identity of formations can never again be called in question by a geologist who do- 

 servos the name. 



Attention is called to the fact that no observer has yet drawn the 

 line of distinction between the Trenton and the newer limestone super- 

 imposed upon it in Ohio and Kentucky, because the similarity in min- 

 eral character and color is so perfect, and both formations thin out into 

 mere seams and layers. It is also stated that the catalogues of organic 

 remains of the Silurian system hitherto published in Europe are cal- 

 culated only to mislead and confound the geological inquirer, as they 

 are far too vague. 1 



In 1840 Mr. Conrad instituted a series of comparisons of formations 

 of the New York section with those of England, based upon the fossils 

 contained in their respectative formations. He compares the Oaradoc 

 sandstone with the limestone of Trenton Falls, and the Wenlock shale 

 with the Rochester shale, and the Wenlock limestone with a limestone 

 developed in the Helderberg Mountains. 2 



In the fifth annual report on the paleontology of New York we find 

 the following remarks applied to correlation of the rocks of the State : 3 



The horizontally and undisturbed condition of the strata have enabled us to trace 

 their sequence or order of superposition with comparative ease and greater accuracy 

 than can always be obtaiued in regions where the formations have been much 

 inclined or distorted. These divisions or series are usually composed of various 

 layers, as compact limestone associated with friable shale, sandstone alternating with 

 argillaceous shale, etc., and these modifications of their mineral constituents are 

 generally accompanied by some variation of the organic contents; new species have 

 been introduced, or more ancient ones have disappeared. But it is only at the junc- 

 tion of two formations that each group of organic remains is uot perfectly distinct 

 and characteristic, a mixture of species sometimes occurring which proves a gradual 

 transition from one era to another, and gives rise to some uncertainty where the 

 exact line of demarcation should be drawn. This fact opposes the idea sometimes 

 indulged by speculative geologists, that sudden convulsions of the earth's surface 

 have been the cause of exterminating forms of life, and the introduction of others to 

 supply their place. The change seems rather due to alteration of temperature in the 

 water, whatever new physical conditions of the earth resulted at the same time. 

 Such phenomenn, however, do not interfere with the general distinctive characters 

 of the stratigraphical divisions adopted in this work. It is now, I believe, an undis- 

 puted point in geology that certain groups of organic remains belong exclusively to 

 certain formations, and that these strata, in a general way, may be known and com- 

 pared by the same groups of genera, if not species, in every region of the earth. It 

 is also established, that whilst some genera and many species are restricted to a sin- 

 gle formation, others have originated at an early period and continued to exist 

 throughout a large portion of the time occupied in the deposition of a system or series 

 of formations. * # * 



The color and even mineral character of a formation usually varies greatly over an 

 extended region, but it may be recognized by its fossil contents ; for example, the 

 black slate of the Mohawk, characterized by a peculiar trilobite, Triarthrus, which 



1 Op. cit., p. 59. 



2 On the Silurian System, with a table of the strata and characteristic fossils. Am. Jour. Sci., vol. 38 

 1840 pp. 87-91. 



3 Fifth annual report on the paleontology of New York. Fifth Annual Report of the Geological 

 Survey oi New York, 1841, pp. 25-27. Albany. 



