282 A. J. JUKES-BROWNE ON THE RELATIONS OP 



I have only been able to find 72 species belonging to undoubted 

 Upper Greensand, viz. that of the Isle of Wight and the Malm rock 

 and Firestone of the Wealden area : 20 of these (or 28 per cent.) are 

 common to the Cambridge fauna ; but most of them are bivalves with 

 a wide range, very few of the Cephalopoda or Gasteropoda being 

 among them, and none of the more characteristic fossils. 



From the "Warminster Greensand about 140 species have been 

 recorded; but only 28 (or 20 per cent.) are also found among 

 the Cambridge coprolites, and, with the exception of JRhynchonella 

 compressa, none of these are characteristic "Warminster fossils ; there 

 is therefore little affinity between the two faunas. 



With regard to the fossils really belonging to the Cambridge 

 Greensand, the numbers given in the above Table clearly demon- 

 strate their relationship with the fauna of the Chalk-marl; eight 

 species are described as peculiar, but nearly all the rest occur in 

 the Chalk-marl or Grey Chalk. These strata, as exhibited near 

 Cambridge, in the Isle of Wight, and at Folkestone, contain an 

 invertebrate fauna numbering about 60 species; of these, 29 (or 

 nearly 50 per cent.) are found in the Cambridge bed. 



§ 3. Conclusions from Palceontological Evidence. — This being so, it 

 follows that a distinct paleeontological break, so to speak, exists 

 between the two elements of the Cambridge fauna, a glance at the 

 preceding Table showing how much more closely the one part is 

 bound to the Upper and Lower Gault than it is to the Chalk-marl. 

 And yet this distant resemblance to the Chalk-marl fauna is greater 

 than its resemblance to that of Warminster or other Upper Green- 

 sand localities ; if, therefore, we cannot ascribe the Cambridge fauna 

 as a whole to the Chalk-marl, still less can we class it with the 

 Upper Greensand. The explanation of the facts I conceive to be 

 that, while conditions were inimical to the immigration of many 

 new forms, a few species found it possible to prolong their existence 

 through the intervening time ; and I regard the course of events as 

 having taken place somewhat in the following order : — Originally 

 there existed over the Cambridge and Bedford area a considerable 

 thickness of Upper Gault, becoming, as usual, more sandy towards 

 the top, and containing a fauna of very similar facies to that of the 

 Swiss Gault superieur or Gres Vert ; continuity of deposition, how- 

 ever, was then interrupted, and in place of any true Upper-Grecn- 

 sand beds being formed, those already existing were subjected to 

 the action of strong marine currents, which sifted out their contents 

 and mixed them up with foreign stones and boulders derived from 

 more northern localities*. 



This period of erosion appears to have lasted during the time that 

 50 or 60 feet of strata containing an Upper-Greensand fauna were 

 being deposited in more southern areas ; but the conditions which 

 introduced such agencies gradually changing, the results of their 

 work would be left on the sea-bottom and eventually would be 

 incorporated with the next succeeding series of deposits, at the 



* See a note " On the Included Rock-fragments of the Cambridge Upper 

 Greensand," by Sollas and Jukes-Browne, Quart. Journ. Qeol, Soc. vol. xxis. p. 1 1. 



