468 E. O. TJLRICH CORRELATION OF THE STRAND-LINE 



scale, the larger divisions up to the periods and eras being hut combina- 

 tions of the minor units. Concisely stated, the beginning or end, as the 

 case may be, of a terminal unit of a si&ge, epoch, or period at the same 

 time delimits the division of higher rank of which it forms a part. 



Lithologic criteria and the vertical range of fossils have hitherto been 

 employed in seeking to fix these boundaries. For many reasons the results 

 have been deplorably indefinite, inharmonious, and often quite inaccurate. 

 The need of some more exactly determinative and finally dominant cri- 

 terion is undeniable. 



Even when we have properly decided that of fossils found in some ex- 

 posure of rocks one lot is of Silurian age and another of Devonian, or, to 

 use progressively smaller units, the first is of Cayugan and under that of 

 Manlius age, and the second lot of Helderbergian, or, more exactly, of 

 Coeymans or possibly of Keyser age, the results still fall short of the most 

 desired practical object of the inquiry, namely, the means employed do 

 not suffice in determining just where in the perhaps barren interval be- 

 tween the accurately identified fossiliferous beds the boundary between 

 the older and the newer series of deposits may be sharply drawn. In 

 short, we require something that will supply the deficiencies of the purely 

 paleontologic and lithologic methods, and thus assure greater defmiteness 

 in the delimitation of stratigraphic and time units. The means is at 

 hand. It lies among those criteria of diastrophism that to a limited 

 extent have always been employed, namely, those indicating alternate 

 advance and retreat — displacement — of the strand-line. 



As defined by me, the criteria of diastrophism embrace all physical 

 and, to a certain extent, all organic phenomena implying horizontal and 

 vertical movements of the crust of the earth; also surface deformation 

 which may aid in the causation of such body movements. Diastrophic 

 processes, therefore, range from the impulsive grand deformations — 

 which may be more local than general in their visible effects — to those 

 much more gentle yet often widely manifested movements which origi- 

 nate through the operation of degradational processes and serve to main- 

 tain the isostatic equilibrium of the shell. Whatever the cause of these 

 body deformations and however manifested, they always tend in some 

 larger or smaller degree to cause displacement of the strand-line. If the 

 movement resulted in a deepening of one of the oceanic basins, increasing 

 its capacity, the waters must correspondingly be universally and simulta- 

 neously withdrawn from the epicontinental basins. If, on the contrary, 

 the capacity of the oceanic basins is diminished by sedimentation, the 

 waters must gradually overflow the land. As in the first case, so in this, 

 the effect on the strand-line is universal and similar on all the continents. 



