IN THEIR ROLE AS WORLD-BUILDERS. 11 



do not exist together, but the transition from one dominant to 

 the other is often quite sharp. We show a section from Sherani, 

 on the N.W. frontier of India, illustrating the junction of the 

 two beds. Within a thickness of two inches the rock turns from 

 a Nummulitic to an Alveoline limestone (PI. 3, fig. 2). What 

 possible explanation can there be for such a radical and cata- 

 clysmic change, necessitating the practical extinction of one 

 dominant, and the sudden rise to prominence of another, widely 

 different, type? It cannot be a case of evolution, as the two 

 species represent entirely different types of structure. 



With the passing of the Eocene period the Foraminifera lose 

 their all-important position as rock-builders. Through Oligocene 

 and Miocene times they continued to flourish, and to form 

 deposits largely or entirely built up of their remains. The genus 

 -Nummulites dies out, dies so completely that at the present day 

 it is represented by only a single small species of rare occurrence 

 in tropical seas. Alveolina persists, but no longer as a dominant. 

 Orbitoides, a highly specialised type which had made its first 

 appearance in the Chalk of Maestricht, attains sudden abundance 

 and forms great beds of Orbitoidal limestone in all the Con- 

 tinents, only to die out absolutely in the Miocene (PI. 3, fig. 3). 

 But the Miocene and later Tertiary deposits, though often 

 presenting an abundant and extremely varied foraminiferal 

 fauna, no longer owe their existence to the occurrence of one or 

 iew species in enormous numbers, except in those comparatively 

 few deep-sea deposits which have been raised to the surface 

 in the West Indies, New Guinea and the Pacific, and which 

 are similar in structure and often in species to the deposits 

 which are being found in the deep sea to-day (25) (26) (PI. 3, 



&s - i] - . .... 



Perhaps the conditions under which foraminiferal life exists 

 to-day may help to explain the change. We have now no seas 

 -swarming with Nummulites and Alveolina, to the practical 

 exclusion of other species Here and there about the world the 

 shallow-water Foraminifera are to be found in such .profusion 

 that, given favourable means of preservation, we should have in 

 time a true foraminiferal limestone. From the shallow waters of 

 the West Indian seas we have received dredgings almost entirely 

 composed of the genera Orbiculina and Miliolina. In the shallow 

 lagoons of the Pacific Tinoporus baculatus, Alveolina Boscii and 



