INTRODUCTION. O 



coarse sand to fine sand, silt, clay, and calcareous mud. 

 But along coasts which have many inlets or bays this 

 order is not so well preserved, and fine silts or muds are 

 sometimes formed quite close in shore ; similar deposits of 

 very fine grain occur also in the estuaries of large rivers. 



In dealing, therefore, with clays and shales we must be 

 guided largely by their fossil contents, and by the nature 

 of the other beds with which they are associated, in de- 

 ciding whether they are shallow or deep-water deposits. 

 Black carbonaceous clays, for instance, containing much 

 organic matter and associated with sandstones, would lead 

 us to suspect the neighbourhood of a swampy shore or the 

 estuary of a large river. 



Limestones, again, though more often formed in com- 

 paratively deep water at a distance from land, are some- 

 times accumulated quite close to a coast-line, like the 

 coral limestones of the present day. A pure limestone 

 may be taken as a proof of clear water, and if there is 

 reason to suppose that it was formed at no great distance 

 from land, it may be assumed that no large rivers issued 

 from the coast in question. 



Every case in which limestones occur must be dealt with 

 on its own merits, and no hasty conclusion as to the dis- 

 tance of land should be formed. There is, in fact, no 

 rock-name that includes so many varieties as limestone, 

 and the conditions under which a limestone may be formed 

 are as numerous as are the varieties. That many of the 

 limestones which occur among British rocks are shallow- 

 water deposits is generally admitted ; no one would claim 

 a deep-water origin for a current-bedded oolitic limestone, 

 or for the Llandovery limestones, which succeed sandstones 

 and conglomerates, or for the Liassic limestones of Gla- 

 morganshire. The Carboniferous limestones of Northum- 

 berland and Scotland can hardly have been deep-water beds, 

 and perhaps none of the Carboniferous limestone, even in 



