268 W. J. Sottas — On Greensand Foraminifera and Sponges. 



V. — On the Foraminifera and Sponges of the Upper 

 Greensanb of Cambridge.^ 



By W. Johnson Sollas, Associate of the Eoyal School of Mines, London, 

 Scholar of St. John's College, Cambridge. 



THE Upper Greensand of Cambridge occurs as a thin but very 

 important bed, covering the eroded surface of the Gault, and 

 blending upwards into the overlying Chalk-marl. It consists essen- 

 tially of Chalk-marl, saturated with green grains of Glauconite, and 

 crowded with a variety of fossils. The fossils in gi-eat part derive 

 their origin from the Gault, out of which they have been concen- 

 trated by a natural process of levigation. At the time when the 

 Gault sea was shallowing to its close, its submarine shores became 

 eroded by a cold current flowing from the north, which bore away 

 its lighter sedimentary clay, and left its imbedded fossils behind, to 

 be subsequently rolled into the sublittoral deposit of the Upper 

 Greensand. Thus it happens that in our neighbourhood the upper- 

 most beds of the Gault are not to be found, and their only represen- 

 tative is this mere pebble-bed at the base of the Chalk. The rich 

 fauna in our formation thus collected ready to the hand of the 

 Palaeontologist has been well worked out in its higher develop- 

 ments by Mr. Barret, Mr. Seeley, and various other observers. Of 

 the Protista and Coelenterata, however, some departments have been 

 left all but untouched ; and it is to the Sponges of the latter and 

 the Foraminifera of the former that my paper refers. The 

 Foraminifera occur abundantly, and in a rich variety, which seems 

 to anticipate their more luxuriant appearance in the succeeding 

 Chalk. The Vitrea perforata are represented by large forms of 

 BuUmina, Textularia, and Orthocerina, which mimic truly arenaceous 

 foraminifera by imbedding the siliceous and volcanic sand of the 

 formation in their tests. Cristellaria, Flabellina, and Frondicularia 

 are numerous, and of very fine growth ; the Frondicularia some- 

 times attaining a ^" in length. 



Rotalina is an abundant genus, so also are Globigerina and Lagena. 

 The occurrence of this latter genus is noteworthy, for with the 

 exception of one species, Z. apicidata found in the Gault on the 

 Continent, it has not been met with before below the Maestricht 

 Chalk ; in the Greensand, however, are several species, both of 

 Ectosolenian and Mitosolenian forms, of which one at least appears 

 to be identical with a living species (Z. squamosa) of the British 

 seas. The large and numerous Trochammina and Lituola constitute a 

 marked feature of the deposit. Of the same size as the Foraminifera, 

 and associated with them, occur those characteristic green grains, 

 whose origin in this formation has so long remained a subject for 

 the merest conjecture. It is well known that ofi' the coast of 

 America, in the JSgean, and in some fossil deposits, green grains have 

 been met with, distinctly derived from casts of Foraminifera. It was 

 denied, however, that this was the case with the green grains of the 



1 Read before the Cambridge Phil. Soc, March, 1873. . 



