1822.] Geology of the Isle of Wight, 8^'c, ' 337 



account might lead any one who is unacquahited with the fossils 

 of the district to suppose that the thin bed of green sand is the 

 representative of the green sand formation ; and consequently, 

 that the great bed of clay (gait) is identical with the Tetsworth 

 clay before mentioned. The conclusion would not, however, 

 bear the test of examination for the following reasons : 



(1.) The fossils of the thin bed of green sand, amounting to 

 about 20 species, do not belong to the suite of the green sand 

 formation; but partake of characters common to the upper and 

 lower beds associated with it. 



(2.) The fossils of the clay {gait) form a suite nearly identical 

 with those of the Folkstone rnarl which reposes on the green sand 

 formation.^ 



(3.) When the gait in the vicinity of Cambridge is perforated 

 for the purpose of obtaining water, the first discharge forces up 

 a considerable quantity of green sand — a fact which indicates 

 the existence of the g7'een sand formation below the galt.f All^ 

 these facts combined with the intimate connexion between the 

 chalk and the beds on which it reposes, lead to the conclusion 

 that the gait of Cambridge is an argillaceous variety of chalk 

 marl. Any system of classification which unites this great 

 argillaceous deposit with the inferior green sand formationy^ 

 assumes the existence of a relation respecting which the denu- 

 dations of the country afford no evidence whatsoever. 



Notwithstanding the intimate relations between the argilla- 

 ceous marl and the superincumbent beds, it would not be expe- 

 dient to represent it on a map by the ordinary colour of the 

 chalk ; for the colour would then cease to mark the boundary of 

 a distinct escarpment. If these views be correct, the proper^ 

 mode would be to represent the argillaceous varieties of chalk 

 marl by a distinct colour ; which would then mark the superfi- 

 cial extent of aflat region stretching out from the foot of a well- 

 defined chalk escarpment. The mode recommended would 

 moreover be in harmony with that which is adopted in the super- 

 ficial dehneation of the greater part of the Enghsh oolite series. 

 This series (as is obvious from the enumeration in Mr. Green- 

 ough's Map) consists of three distinct oolitic deposits, each rest- 

 ing on a great bed of clay. The l^ias c/^j/and the Oxford clay 

 have appropriate colours assigned to them. In conformity with 

 the system, an appropriate colour ought also to be given to the 

 Kimmeridge clay wherever it appears at the surface. The Port- 

 land oohte and the superincumbent Purbeck beds might then be 

 conveniently represented by a single tint. 



* Geol. Trans, v. 5T. 



-|- The supplies of water obtained by boring have never been known to fail. This 

 seems to prove that there is an impervious bed, probably of clay, immediately under the 

 green sand on which the gait reposes. For it is hardly conceivable that a large forma- 

 tion of sand should be always so saturated with water as to be able to force up a colunm 

 to the heigkt of nearly 200 feet, wherever the upper bed is perforated. 



Neiv Series, vol. iii, z 



