52 DESCRIPTION OF THE STRATA. 



in the southern counties; for the latter is of a hard quality; but it 

 does not appear to differ much from the argillaceous chalk or chalk 

 marl, found below the hard chalk in these counties. It may be ob- 

 served, however, that our white chalk is of a harder quality than some 

 of the upper chalk of the south. The coloured chalk occvu-s beneath 

 the white chalk in Lincolnshire, as well as in our district. 



The remarkable conformation of these rocks, in having the upper 

 and middle part massive, while the lower part is in thin flat strata, is 

 not peculiar to tlie chalk hills. The same thing occurs in the mag- 

 nesian limestone of the county of Durham; particularly at the Mars- 

 den rocks, between Sunderland and Shields. There the great body of 

 the limestone appears massive, while below it there is a limestone of 

 a different kind, arranged in thin strata, and even in some places 

 finely laminated, its thin lamina when separated formiugjlexible lime- 

 stone. The line between the upper and lower limestone may be dis- 

 tinctly traced; especially as the lower, being softer than the mass 

 abo^e, is often washed away by the sea, while the upper is left as a 

 roof over the caverns thus excavated. A similar appearance may be 

 observed in the hard porphyritic-looking rocks at St. Abb's Head in 

 Berwickshire, which are usually designated by the harsh sounding 

 name grawacke. The lower rocks form flat strata shelving towards 

 the sea, while above them are lofty and massive rocks, rising in a 

 kind of towers or columns, parted by vertical fissures. 



Having described the chalk itself, it will be necessary before pro- 

 ceeding to the next strata, to take notice of some substances that are 

 imbedded in it. Its organic remains, which are not very numerous, 

 especially in the coloured chalk, will fall to be considered in another 

 division of our Work. Here we wovdd advert to minerals of a different 

 ■description. 



A soft grey marl resembling in colour the lowest beds of the grey 

 «halk, is found in the Assures of the upper chalk; especially in the 



