J. L. LOBLEY — CRETACEOUS ROCKS OF ENGLAND. i 



It is a marine deposit, the fossils shells abundantly found in it being 

 the remains of genera Kving only in salt water, as, for instance, 

 the Trigonia and the Terebratula, both of which now flourish in 

 Australian seas. It will be seen from this illustration how im- 

 portant for the right reading of the history of the formation of the 

 earth revealed by the rocks is a knowledge of their fossils, which 

 can tell us whether the water in which a rock was deposited was 

 fi-esh, or salt, or brackish, or, in other words, whether the deposition 

 took place at the bottom of a lake, a sea, or an estuary. The 

 Lower Greensand is especially well seen in the Isle of Wight, the 

 sea-cliffs east and west of Shanklin presenting a fine section, while 

 the cliffs west of Black Gang Chine exhibit the many beds into 

 which the whole series has been divided, each characterized by 

 peculiar species of fossils. 



The third great division of the Cretaceous rocks in ascending 

 order is the Gatjlt, an important bed of stiff blue clay that may be 

 studied to advantage at Folkestone, and is also well seen near 

 Cambridge. The thickness of the Gault is perhaps 200 feet, some 

 beds, especially near Folkestone, containing abundant and generally 

 beautifully preserved fossils. Amongst these the Jmmonites are 

 conspicuous. Ordinary univalve shells, or Gasteropoda, are 

 numerous and varied, and the remains of what may be said to be 

 the progenitors of our crabs and lobsters are frequently met with. 



To the beds of sands succeeding the Gault the name of the 

 Upper Greensand has been given. These sands are generally 

 characterized by the green grains previously mentioned, and seldom 

 display those rich brown colours usually exhibited by the Lower 

 Greensand. These Upper beds are of much less importance than 

 the Lower, being by some considered to be a mere basement bed of 

 the Chalk, and vary greatly in thickness, diminishing northwards 

 from about 140 feet in Wiltshire to not more than two feet near 

 Cambridge, where they form the famous " Coprolite " beds that 

 yield so abundantly those remarkable phosphatic nodules, the high 

 agricultural value of which was discovered by the late Professor 

 Henslow. In addition to the phosphatic bodies, these thin beds 

 contain many fossils, including the remains of most extraordinary 

 creatures which were neither birds nor reptiles, but possessing 

 characters common to both, being in fact flying lizards, and of no 

 inconsiderable size, some having had an expanse of wing of 20 feet. 



We now approach the most conspicuous, the most important, and 

 in every respect the most remarkable member of the Cretaceous 

 group of rocks, the Chalk, a rock specially interesting to the 

 members of this Society, since the Chalk forms nearly the whole of 

 Hertfordshire, and is the formation underlying the towTi of Watford. 

 This soft white rock, so welcome to the eye of the returning 

 traveller when he sees it forming the sentinel-like "Needles," or 

 the towering bulwarks of the Cliffs of Dover, is as remarkable in 

 its origin as in its aspect, and tells a tale to those who can read 

 the records of the rocks aright of marvellous interest and value. 



Probably no geological formation so conspicuously affects the 



