174 



LYELL'S ELEMENTS OF GEOLOGY. 



Older Pliocene Strata English Crag. 



the upper of which may be termed the red, and the lower the 

 coralline crag.* The inferior division, however, is of very 

 limited extent, ranging over an area about twenty miles in 

 length, and three or four miles in breadth, between the rivers 

 Aide and Stour. 



The red crag is generally at once distinguishable from the 

 coralline, by the deep red ferruginous or ochreous-colour of its 

 sands and fossils. Its strata are also remarkable for the oblique 

 or diagonal position of the subordinate layers (see p. 32.) ; and 

 these often consist of small flat pieces of shell, which lie parallel 

 to the planes of the smaller strata, showing clearly that they 

 were so deposited, and that this structure has not been due to any 

 subsequent rearrangement of the mass after deposition. That 

 the ancient sandbanks in question had sometimes sides sloping in 

 all directions, is implied by the fact that the oblique layers some- 

 times slant towards all points of the compass in different parts 

 of the same quarry. They were probably shifting sands, and a 

 great proportion of the shells composing them have been ground 

 down to small pieces, while others have been rolled ; and the 

 two parts of the bivalves are almost invariably disunited. The 

 red crag contains some peculiar fossils, and others which seem to 

 have been washed out of the lower or coralline crag. Some 

 few of the bivalves of the red crag are entire, with both valves 

 joined. 



The coraUine crag is usually free from ferruginous stains, and 

 consists of light greenish shelly marl, and white calcareous sand. 

 Sometimes it forms a soft building stone, in which entire shells, 

 echini, and many zoophytes are imbedded. Here and there the 

 softer mass is divided by thin flags of hard limestone, in which 

 are corals in a good state of preservation, which evidently grew 

 at the bottom of a tranquil sea, in the position in which we now 

 see them. Yet the sands in this formation, as in the red crag, 

 are often composed entirely of comminuted shell. 



In some places, as at Tattingstone, in Suffolk, the lithological 

 distinction of the two divisions, though perceptible, is much less 

 marked ; the inferior crag being composed chiefly of greenish 

 marl, with only a few stony beds. The shells also are mostly 

 broken, and corals are almost as rare as in the red crag. 



At some places, as near Orford, the coralline crag is exposed 

 at the surface, and the bottom of it has not been reached at the 

 depth of fifty feet. Yet not far from this town, the surface is 

 occupied exclusively with red crag, which rests immediately upon 

 the London clay. Wherever the two divisions are found toge- 

 ther, the coralhne mass is the lower of the two, and is interposed 



* London and Edin. Phil. Mag. No. 38. p. 81. Aug. 1835. 



