IMPERFECTION OF PAL^EONTOLOGICAL RECORD. 67 



it is certain that there 'must be for every bed a point whence 

 the largest amount of sediment was derived, and in the neigh- 

 bourhood of which the bed will therefore be thickest. Thus, 

 if we take a series of beds, such as sandstones and conglo- 

 merates, which are the product of littoral action, and are de- 

 posited 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 lime- 

 stone. The beds of limestone would at first be very thin, 

 but in proceeding still in the direction of 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 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 off or " thinning out " in all directions towards the 

 circumference (fig. 11). 



Fig. 11. Diagram to show the " thinning out" of beds, a, Sandstones and 

 Conglomerates ; b, Limestones. 



In a general way this holds good, not only for any partic- 

 ular 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 conveyed, or where 

 the other conditions of accumulation were especially favour- 

 able. At this point, therefore, the beds are thickest, and 

 from this they thin out in all directions. It need scarcely 

 be pointed out, indeed, that some such state of things is un- 

 avoidable in the case of every bed or group of beds, since no 



