Parr L] CHRONOLOGICAL VALUE OF STRATA. 497 
of the Mississippi, and to have been completely buried in sediment 
before decay. | 
Continuous layers of the same kind of deposit suggest a per- 
sistence of geological conditions; numerous alternations of different 

Fic. 215.—Ererct TREE-TRUNK RISING THROUGH A SUCCESSION OF STRATA, 
KILLINGWORTH COLLIERY, NEWCASTLE (B.). 
_ a, High Main Coal-seam ; b, bituminous shale; e¢, blue shale; d, compact sandstone ; 
e, shales and sandstones; /, white sandstones; g, micaceous sandstone; h, shale. 
_kinds of sedimentary matter point to vicissitudes or alternations of 
conditions. As a rule, we should infer that the time represented by 
a given thickness of similar strata was less than that shown by the 
same thickness of dissimilar strata, because the changes needed to 
_, bring new varieties of sediment into the area of deposit would 
usually require the lapse of some time for their completion. But 

Fic. 216.—Erecr Tree-Trunk (@ a) IMBEDDED IN SANDSTONES (¢ ¢c) AND SHALES 
(d d), 1ts INTERIOR FILLED WITH DIFFERENT SANDY AND CLAYEY STRATA, AND THE 
WHOLE COVERED BY A SANDSTONE BED (6) (B.). 
_ this conclusion might often be erroneous. It would be best supported 
_ when, from the very nature of the rocks, wide variations in the 
| character of the water-bottom could be established. Thusa group of 
| shales followed by a fossiliferous limestone would mark = period of 
K 

