GEOLOGY. 451 



turbed sediments aloog the whole eastern border of the basin, consti- 

 tuting the First Graywacke and the Sparry Lime-rock of Eaton; being 

 the Upper Taconic of Emmons, and the Potsdam and Quebec groups 

 of Logan. The Hudson -River group, as originally defined, included 

 the whole of the Cambrian Appalachian, besides some of the under- 

 lying Taconian slates and portions of overlying Ordovician beds, of Lo- 

 raine age, in consequence of which the name of Hudson-River group 

 came to be regarded as the paleontological equivalent for the Loraine. 

 The Adirondack area of the Cambrian includes the stable and little-dis- 

 turbed area around the Adirondack Mountains, embracing the Chain- 

 * 

 plain and Ottawa Basins, in which the series is represented only by the 



Potsdam and Calciferous divisions, corresponding apparently to but a 

 small portion of Cambrian time. The physical conditions of the Mis- 

 sissippi area, as seen in the valley of the upper Mississippi, appear to 

 have been similar to those of the Adirondack region. The region of 

 the Cordilleras, in which great developments of Cambrian rocks are 

 met with, presents conditions of deposition unlike the other. While in 

 the Adirondack area there is a break, both paleontological and strati- 

 graphical, between the Cambrian and the Ordovician, which begins in 

 some places with the Chazy and in others with the Trenton, we have, 

 according to the late studies of Walcott in Nevada, a gradual passage 

 from the Cambrian to the Ordovician (Lower Silurian or second fauna of 

 Barraude). "In this section," he remarks, " wehave an illustration of the 

 gradual extinction of an older fauna as a new one is introduced, the 

 sedimentation continuing, and no physical disturbance occurring to 

 change the conditions of animal life." The break between the Calcifer- 

 ous and the Chazy is here filled. It is to be remarked in this connec- 

 tion that the fossils of the Levis limestone of Canada (the Sparry Lime- 

 rock of Eaton) were long since declared by Billings to occupy an inter- 

 mediate position, and constitute a passage from the Calciferous to the 

 Chazy. From the incomplete data which we now possess with regard 

 to the lower Paleozoic rocks of the northwest side of Newfoundland, 

 there is little doubt that further studies there will add to our knowledge 

 of the relations of the first and second faunas, and help to illustrate the 

 conception of an unbroken succession. The notion that breaks, uncon- 

 formities, and sudden transitions should form the basis of classification 

 in stratified rocks, is growing obsolete. (See further, for details of the 

 Paleozoic rocks of the Cordillera region, the account of the Geology of 

 the Eureka district in Nevada.) 



In the Grand Canon of the Colorado there is found immediately below 

 the Devonian a series of Cambrian strata, known as the Tonto group, 

 containing an abundant fauna like that of Mie Potsdam of the Missis- 

 sippi area. This group rests unconformably upon a vast series of uu- 

 crystalline shales, sandstones, and limestones, measuring over 11,000 

 feet, and including 1,000 feet of interbedded igneous rocks, constituting 

 the Grand Canon and Chu-ar groups of Powell. These have afforded 



