TRIPARTITE SERIES. 393 



above Banbury. We have therefore sea deposits under ordinary 

 quiet conditions, probably not far from shore, and in some cases 

 even littoral, the materials being derived from wasting lands and 

 organic accumulations. 



These materials are principally calcareous, arenaceous, and argil- 

 laceous, with admixture or impregnation of iron oxides, carbonates, 

 and sulphides. 



The three principal mineral elements are arranged in ternary 

 groups, so that in many places the order runs thus, downwards a : 



Calcareous .... Portland oolite. 

 Arenaceous . . . . Portland sand. 

 Argillaceous .... Kimmeridge clay. 



Calcareous .... Coralline oolite. 

 Arenaceous .... Calcareous grit. 

 Argillaceous .... Oxford clay. 



Calcareous .... Cornbrash. 



Arenaceous .... Hinton sand. 



Argillaceous . . . . Bradford clay and Forest marble. 



Calcareous .... Great oolite. 

 Arenaceous .... Stonesfield sands. 

 Argillaceous .... Fuller's-earth. 



Calcareous .... Inferior oolite. 

 Arenaceous .... Midford sands. 

 Argillaceous .... Upper lias. 



The remarkable ternary order here so often repeated is found 

 again in the sections of carboniferous strata which include the three 

 terms in Yorkshire. As many as five of these combinations can 

 sometimes be found in the Yoredale series alone b . It is repeated 

 in the cretaceous series chalk, greensand, and gault and the 

 explanation has been furnished in a discussion of the Silurian 

 Strata c , on the simple and sure basis of interrupted depression of 

 the sea-bed. (See pp. 92, 93 of this volume.) 



In the cases before us the liassic sea-bed first receives only the 

 finest sediments which can fall in deep water ; by degrees these 

 sediments accumulate so as to bring the sea-bed near enough to 



a Thinner sands occur in some places above the Portland and Coralline oolites. 

 b See Illustrations of the Geology of Yorkshire, vol. ii. 

 c See Memoirs of the Geological Survey, vol. ii. 



