216 PICTURESQUE SKETCHES 



this series, without special reference to those beds 

 in which they seem to us now to be most abundant, 

 because I doubt whether in many cases we are at all 

 able to decide upon such a point for want of evidence. 

 The time during which the oolitic period lasted 

 was, however, sufficient for the destruction of many 

 species, and the introduction of an equal or greater 

 number of new ones ; and I only question the fact 

 of any sudden or considerable change of this kind 

 being distinctly indicated by the fossils of any 

 known locality. 



The great deposit of calcareous mud, which has 

 been described under the name of lias, and which 

 is so richly loaded with interesting fossils, does 

 not pass by any sudden change into more calca- 

 reous beds, and so into limestone, but first of all 

 seems to have become more sandy. The sand is 

 often loose and micaceous, and it probably assisted, 

 by forming a sea-bottom so entirely different from the 

 lias, to change the inhabitants of that sea-bottom, 

 and produce the necessity for the introduction of 

 a distinct group. This is the more probable, since 

 we find that the free swimming animals, such as the 

 belemnites, were little or not at all altered, while the 

 upper lias sands contain but few fossils of attached 

 animals, and these afterwards give way to oolitic 

 species. 



The first change, therefore, announcing the intro- 

 duction of the oolitic period may have been a depres- 

 sion of the coast line of the liassic sea, altering the 

 conditions of life by changing at once the depth and 

 the nature of the sea-bottom, and permitting a de- 

 posit of sand to cover the clays of the lias as yet 



