30 INTRODUCTION. 



thickness, differs altogether from that of the beds immediately 

 above and below, and is characterised by species peculiar to 

 itself. In such cases we can only suppose, that though no 

 physical break can be detected, the deposition of sediment was 

 interrupted by pauses of incalculable length, during which no 

 additional material was added to the sea-bottom, whilst time 

 was allowed for the dying out of old species and the coming in 

 of new. The incessant repetition of such intervals of unrepre- 

 sented time throughout the whole stratified series is convincing 

 proof that the palaeontological record is, and 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 

 technically 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 therefore be thickest. Thus, if we take a 

 series of beds, such as sandstones 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 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 



Fig. 4. Diagram to show the " thinning out " of beds, a Sandstones and 

 Conglomerates ; b Limestones. 



would at first be very thin, but in proceeding still in the direc- 

 tion 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 to- 

 wards the circumference (fig. 4). 



