Shaler.] 412 [Dec. 7, 



they cannot be due to alterations in the nature of the sediments 

 brought to the bottom by changes in the movement of tidal or 

 other oceanic currents. 



If we examine a typical section of stratified rocks, we per- 

 ceive that beside the alteration in the physical character of the 

 deposit is in most cases an important coincident change in its fossil 

 contents. In the greater number of cases these clay partings are 

 destitute of fossils. If they contain organic remains they are 

 generally, in part, at least, of other species than those in the lime- 

 stones which lie above and below. In many cases it can be ob- 

 served that the limy matter slowly reappears in the layer above 

 the clay, though in most cases it suddenly disappears in the layer 

 below. It may thus often be noted that the upper part of a layer 

 of a limestone abounds in well preserved fossils which project 

 slightly from its surface, while the lower layer of the overlying 

 limestone stratum exhibits no such distinct fossils. Those which 

 it contains are more or less commingled with the clay matter. 



At fii'st sight the clay element of the partition between adjacent 

 beds of limestone appears like a new material introduced into the 

 section, but if we proceed to dissolve in dilute acid the lime from 

 the layers of limestone, we find at the end of the process a resid- 

 uum which is essentially like that composing the clay partitions. 

 It is evident that the whole section is composed of a continu- 

 ously formed deposit of clayey matter in which the lime element 

 occasionally attains such prominence that the clay is hidden from 

 ordinal observation. The clay is the steadfast element in the 

 deposit ; the lime laid down by the organic forms, the invariable 

 element therein. 



If the above conclusions are correct, then it is necessary for us 

 to account for the sudden variations in the organic life on the sea 

 floor which has led to the changes in the proportion of the limy 

 element of the sediments. It is evident that this variation in the 

 organic element of the rocks can best be explained by some cause 

 or causes which may have led to the sudden death of the organic 

 species, and, after a time, during which they did not contribute 

 their remains to the strata, a tolerably swift reintroduction of the 

 forms. As before noted, the lower portion of each bed commonly 

 exhibits a somewhat gradual transition from the condition of clay 

 to that of limestone, indicating a gradual passage from one state 

 of the bottom to the other, while the upper part of the layer shows 



