8o8 PRINCIPLES OF STRATIGRAPHY 



5a. Maine land lobe, along the northwesern boundary of Maine, 



6. Gasps sea lobe, a pronounced lobe, the trend actually chang- 

 ing to southeast (40°). 



6a. Cape Breton land lobe, the change of trend being near Syd- 

 ney, Cape Breton, the trend again turning northeastward and con- 

 tinuing thus through Newfoundland. 



19. Domes and Basins. Domes are shortened anticlinal struc- 

 tures with the dip of the strata away from the center in all direc- 

 tions or qnaquaversal. These dips may vary greatly in different 

 domes. In some cases they are so low as to be scarcely or not at 

 all perceptible (Cincinnati dome); in others they may be 45° or 

 over (Black Hills dome). Many of the low-dipping domes are 

 perceptible as such only by the erosion which has removed their 

 central portion, often leaving a topographic depression. Such low 

 domes have also been called par mas, after one of the low east and 

 west ranges which project from the western side of the Urals 

 (which have a north-south trend), and which are formed by gently 

 folded strata, the folds dying out in the plains. 



Basins are the reverse of domes, the strata all dipping toward 

 the center. As a rule, basins are composed of gently dipping strata 

 only so that their basin character is recognized only by the rimming 

 outcrops of the lower strata after erosion (Michigan basin, Paris 

 basin, etc.). Between two basins lies generally a more sharply 

 marked anticline, while between two domes a pronounced syncline 

 often occurs. Sometimes the basin structure is ascertained by the 

 location by borings all over the area of the summit (or bottom) of 

 a certain formation, such as a coal bed or a marked sandstone. 

 Thus the basin structure of Iowa is beautifully brought out by the 

 series of contours connecting areas of equal depression beneath the 

 surface of the summit of the St. Peter sandstone. (Iowa Geol. 

 Survey, Vol. \1, p. 316, map.) 



Eastern North America is marked by a number of distinct basins 

 and domes, many of which are indicated by the outcrops, while 

 others are recognized only from their general relationship and the 

 occurrence of separating anticlines or synclines. All of these basins 

 and domes owe their final character to the Appalachian folding, but 

 some of them apparently existed during much of Palaeozoic time. 

 The accompanying map (Fig. 192) shows the location of these 

 domes and basins. It will be observed that the outermost basins 

 are generally embraced by convex lobes of the Appalachian system, 

 while the concavities of that system are opposite domes or opposite 

 anticlines separating basis. (See also Ruedemann-22 ; Willis-30. ) 



The principal basins so far determined are in the northeast, the 



