320 THE STRUCTURE AND SUCCESSION AT 



depths. The advent of this permanent period of sinking 

 marks the beginning of the formation of the Millstone Grit 

 series. 



Ironstone. — Between the Limestone and Grit is a bed of 

 impure ironstone. It is possible that this was formed in part 

 later than the adjoining beds as a bedded replacement deposit. 

 The limestone being more soluble than the overling grit, would 

 gradually dissolve away by water following the contact line- 

 The grit is a coarse grained permeable rock, w^hile the limestone 

 is of much finer texture. Wat-'r leached from the overlying 

 beds, then, soaked down and overspread the limestone surface, 

 carrying away lime in solution. Iron compounds already 

 existed in the limestone, while additional quantities would be 

 brought in from the grit above. These were deposited in the 

 form of iron oxide in the place previously occupied by lime. 

 This is apparent from the fact that all the calcareous sandstones 

 are red or brown from iron, while the overlying grit is gray as 

 if leached ; and also that the rock containing the iron is silicious 

 to clayey, similar to that containing the lime. 



MILLSTONE GRIT. 



Cross-bedded scmdstones. — With the introduction of the 

 Millstone Grit, variety ceases. In one or two cases an irregu- 

 larity occurred, sufficient to accumulate beach detritus, which 

 now appears as small beds of conglomerate. In general, how- 

 ever, sand was laid down in shallow water. It graduated from 

 coarse to fine, both laterally and vertically, but the change was 

 seldom abrupt. A few sudden depressions succeeded periods of 

 rest, as is seen in comparatively fine sand resting on conglom- 

 erate. False bedding is common, and nearly always in the 

 direction of the dip. Shore phenomena are also evident, from 

 horizontal stems of Calamites, Lepidodendron, and Sigillaria, 

 where they were probably thrown down as driftwood as at the 

 present day. Now and again tangled masses of leaves and 

 twigs occur as if brought there by some current. 



