120 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Where erosion has exposed the interior, however, a very different structure is exhibited. 

 The core of the dome appears to be composed of a brecciated mass formed of hmestone frag- 

 ments bound together by calcareous material that seems to have been a mud derived from the 

 wear of the rocks themselves. Although the base of the formation has never been seen imme- 

 diately beneath one of these prominences, yet from all that can be ascertained from the study 

 of the lower beds in the vicinity, it is probable that the basal strata are homogeneous and 

 horizontal, and unaffected by the pecuharities that lie above. 



In addition to the foregoing peculiarities of stratification, the unity of the formation in 

 eastern Wisconsin is interrupted by ashaly stratum in its subcentral portion, and in north- 

 western Wisconsin by a sandstone layer of very irregular thickness, sometimes merely consisting 

 of a layer of lenticular or pocket-like deposits. Locally there are seams of sand and shale 

 intercalated in the series, particularly in the basal portion. 



The bedding of the rock is usually uneven and heavy, and its texture is coarse, rough, and 

 irregular, though exceptionally it becomes uniform in bedding and grain, forming a beautiful 

 and serviceable rock. 



In composition it is, in the main, a magnesian hmestone, or, more technically, a dolomite. 

 But it contains in addition much siliceous material, most commonly in the form of flint or 

 chert nodules but sometimes in disseminated sand grains and not infrequently as an oohte, 

 in which the center of the spherules is a grain of sand, about which concentric layers of cal- 

 careous material are gathered. Silica also often constitutes a crystalline Hning of cavities, 

 forming beautiful little geodes hned with variously colored quartz. A varying amount of 

 aluminous impurity is also present throughout the rock. 



A notable feature of the formation is the occurrence, at various heights, of brecciated layers 

 interstratified with others more homogeneous. These are composed of fragments of Hmestone 

 in a matrix of finer material, derived from the same source. They seem to indicate that the 

 formation, during its deposition, suffered successive variations of conditions, from comparative 

 quiet to forcible wave action, probably due to shght oscillations of level. The comparative 

 absence of fossils under such circumstances is not surprising. There seems Httle doubt, how- 

 ever, that the material of the formation was derived mainly from the calcareous remains of life. 



Chamberlin and later writers have described the St. Peter sandstone, which 

 overlies the Prairie du Chien group ("Lower Magnesian limestone") as Ordovician. 

 (See p. 180.) Walcott*^^'' quotes the descriptions of the Cambrian strata by Irving, 

 Strong, Wooster, and Winchell, in addition to the account by Chamberlin. The 

 descriptions of Chamberlin have been supplemented in later work by more extended 

 investigation and correlation of the faunas by Sardeson"*" and by Ulrich.*^' 



K-Ii 18. ADLRONDACK KEGION, NEW YORK. 



The Potsdam sandstone of the Adirondack region has been recently studied 

 by Cushing/"^^ from whose detailed account the following passages are taken: 



Lying unconformably on the old and much eroded pre-Cambric surface, a great sandstone 

 formation appears, on the north and east and on the eastern half of the southern border of the 

 Adirondack region. This is a water-deposited formation and, so far at least as its upper por- 

 tion is concerned, a marine formation. It is thickest on the northeast, thinning out to disap- 

 pearance both to the south and west. As, furthermore, it appears to be the upper beds which 

 persist and the lower ones which disappear in these directions, it seems certain that, so far as 

 the immediate region is concerned, the marine invasion came on it from the northeast and 

 extended progressively southward and westward. 



In Clinton County, where the formation is thickest, the basal portion is rather sharply 

 differentiated from the rest in character, and this portion has considerable thickness, though 



