INTRODUCTION. 3 



rence are conglomerates and breccias recalling those which 

 are formed along modern beaches. But in many cases the 

 position of the land tracts during a given period can only 

 be inferred from general considerations, such as the 

 changes in the lithological characters of the sediments, the 

 thinning or thickening of beds in certain directions, their 

 entire absence in certain areas, their conformity or uncon- 

 formity to the underlying rocks. 



Shallow-water Beds. In the first place, therefore, we 

 must know what kinds of rock are likely to have been 

 formed in shallow water. As a general rule, the coarser 

 the grain of the rock the shallower was the water in which 

 it was formed, and the nearer was the shore from whence 

 the component materials were derived, or along which they 

 were moved. Conglomerates consisting of stones which 

 could only be moved by the beating of waves on a shore 

 are, of course, decided evidence of the close neighbourhood 

 of land to the spot where we now find them, and if they 

 contain large angular or sub-angular boulders we may 

 infer that they have been formed below a line of cliffs, and 

 further, if such deposits form a mass of considerable thick- 

 ness, it is probable that much of the material was brought 

 down by floods and torrents from a range of hills above 

 the coast-line. 



We must remember, however, that such coarse deposits 

 are not formed everywhere along a line of coast, and that 

 sandstones may have been formed as close to a shore as 

 conglomerates ; nay, some sandstones have, doubtless, been 

 formed above high water, and were originally sand-dunes, 

 such as are common on our present coasts. 



Sandstones formed from seolian drift, or blown sand, 

 may generally be distinguished from water-borne sands by 

 the character of their component quartz grains ; those of the 

 former being all much worn, rounded, and polished, while 

 in the latter a large proportion of the grains are angular. 



