846 PRINCIPLES OF STRATIGRAPHY 



5. The Basin. _ 



The basin is the complement of the dome, representing the down- 

 ward arching of the strata. As in the dome, the basin may be 

 gentle, with strata so sHghtly incUned as to seem horizontal or it 

 may be a pronounced one with highly inclined sides. The former 

 is represented by the Paris Basin and the Michigan Basin and is 

 in many respects the most, significant type to the stratigrapher, being 

 often difficult to detect. The Paris Basin represents a case in which 

 the successive strata have been breached by radial consequent 

 streams running from the surrounding higher old-land to the lower 

 center, while their tributaries carved out circumferential valleys 

 bounded by outward facing cliffs of the "inface type." The dis- 

 sected basin at this stage differs from the dissected dome in having 

 its oldest formations on the outside, the circumferential valleys 



Fig. 219. Strata of a basin, trim- Fig. 220. The same series after a 

 cated and covered by horizontal second folding and truncation, 



strata. 



being cut out of higher and higher strata toward the center, while 

 in the dome the youngest formations are on the outside, and the 

 successive circumferential valleys are cut on lower and lower strata 

 toward the center. In the basin the in faces or escarpments face 

 outward ; in the dome they face inward. The Paris Basin is most 

 probably to be regarded as a region in the second cycle of erosion, 

 having been peneplained once, after which the topography has been 

 revived by a resumption of stream activity. 



The Michigan basin forms an interesting example of a com- 

 pound type. At the beginning of Devonic time, the basin was 

 formed, after which erosion beveled ofT the margins, leaving the 

 successive formations superimposed after the manner of a nest of 

 plates, the highest being the smallest, while the edges of the suc- 

 cessively lower ones project beyond the higher. Across this series 

 were deposited the Devonic and later strata, after which a second 

 downward arching took place, followed by beveling of the edges of 

 this later formed basin. Thus the highest formations occupy the 

 center of the area and are surrounded by the rims of successively 



