388 THE IMPERFECTION OP THE 



old species, and the coming in of new ones. The incessant repetition 

 of such intervals of unrepresented time throughout the whole strati- 

 fied series is convincing proof that the palseontological record is, and 

 ever will be, a most fragmentary collection of the remains of the 

 animal life of the globe. 



III. Thinning out of Beds : — Another cause by which the con- 

 tinuity of the palseontological record is afiected is what is technically 

 called the "thinning out" of beds. Owing to the mode in which 

 sedimentary beds are produced, it is certain that there must be for 

 every bed a point whence the largest amount of the sediment was 

 derived, and in the neighborhood of which the bed will, therefore, be 

 thickest. Thus if we take a series of beds such as sandstones and 

 conglomerates, which are the products of littoral action, and are 

 deposited in shallow water near a coast-line, it will be found that 

 these gradually decrease in thickness or " thin out," as we pass away 

 from the coast in the direction of deep water. On approaching deep 

 water, however, we might find that though the sandstones were 

 rapidly dying out, the thickness of the entire series might still be 

 preserved, owing to the commencement now of some deep-water 

 deposit, such as limestone. The beds of limestone would at first be 



Diagram to show the "thinning out" ofbeds. a. Sandstones and Conglomeratea. 6. Limestones. 



very thin, but in proceeding still in the direction of deeper and 

 deeper water, we should find that they would gradually expand, till 

 they reached a point of maximum thickness, on the other side of 

 which they would again gradually thin out. Each individual bed, 

 therefore, in any group of stratified rocks may be regarded as an 

 unequal mass, thickest in the centre and gradually tapering ofi" or 

 "thinning out" in all directions towards the circumference. 



In a general way, this holds good not only for any particular bed, 

 but for any particular aggregation or group of beds which we may 

 choose to take. In the case, namely, of every group of beds there must 

 have been a particular point whither sediment was most abundantly 

 brought, or where the other conditions of accumulation were especially 

 favourable. At this point, therefore, the beds are thickest, and from 



