§ 17. MIOCENE, OR MIDDLE TERTIARY. 223 



of which were coteinporaneous with the Crag, while others 

 belong to a more ancient epoch. These beds have resulted 

 from the waste of the secondary rocks, of the Apen- 

 nines, which were dry land before these strata were 

 deposited. 



17. Miocene, or middle tertiary. — The Crag. The 

 miocene are defined as strata containing but a small pro- 

 portion of recent species of shells, seldom exceeding 20 

 or 30 per cent. ; but there are exceptions to the rigid 

 application of this rule in many of the strata comprised in 

 the middle tertiary. Near Bordeaux, and in Piedmont, 

 Hungary, and other parts of the continent, groups of 

 miocene strata occur, and have been described by Mr. 

 Lyell, and other geologists.* 



In England a very interesting assemblage of pliocene 

 and miocene strata is spread over considerable areas, alon^ 

 the maritime parts of the eastern counties, Essex, Suffolk, 

 and Norfolk, — and is called the Crag ; a provincial term, 

 signifying gravel. These beds extend along the coast forty 

 or fifty miles, and form a tract which in some parts is ten or 

 twelve miles in breadth {see Plate I.). The foundation rock 

 of that part of England is the white chalk, which is more or 

 less covered by the London eocene clay, on which the lower 

 Crag is superimposed. The Crag strata consist of loam, 

 clay, sand, and gravel, containing beds of marine shells and 

 corals ; many of the layers being wholly made up of shelly 

 and coralline detritus. In the upper part of the series there 

 are intercalations of fluviatile deposits ; and over all a bed of 

 silt, containing mammalian remains, which insensibly blends 

 with the superficial alluvium of the country. 



Mr. Edward Charlesworth, to whom belongs the merit of 

 having first accurately investigated and interpreted the 

 zoological characters of the Crag, and pointed out the 



* See Elements of Geology, vol. i. chap, xiv: 



