72 



INTRODUCTION. 



ever must be, a mere excerpt from the biological annals of the 

 globe. 



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

 continuity of the palaeontological record is affected is what is tech- 

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

 which sedimentary rocks are produced, it is certain that there must 

 be for every bed a point whence the largest amount of sediment 

 was derived, and in the neighbourhood of which the bed will there- 

 fore be thickest. Thus, if we take a series of beds, such as sand- 

 stones and conglomerates, which are the product 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 approach- 

 ing 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 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 



Fig. 16. — Diagram to show the " thinning out" of beds, a, Sandstones and 

 Conglomerates ; b, Limestones. 



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 direc- 

 tions towards the circumference (fig. 16). 



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 abun- 

 dantly conveyed, or where the other conditions of accumulation were 

 especially favourable. 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 unavoidable in 

 the case of every bed or group of beds, since no sea is boundless, 

 and the sedimentary deposits of every ocean must come to an end 

 somewhere. 



An excellent example of the phenomena above described may be 



