FIELD GEOLOGY. 



fluviatile or marine. When exhibiting "ripple- 

 marks,"*' it indicates a shore-line ; when bound to- 

 gether by a cementing material, it becomes Sand- 

 stone, or, if coarse-grained, Grit. 



Gravel consists of pebbles derived from neigh- 

 bouring rocks, angular or rounded according to the 

 degree of trituration to which they have been sub- 

 jected. It is formed by rivers, or by marine cur- 

 rents at no great distance from the shore. 



When bound together by a cementing material, 

 it becomes, if the pebbles be more or less rounded, 

 a Conglomerate, or Pudding-stone; if the frag- 

 ments be perfectly angular, a Breccia in either 

 case representing a shore-line. 



Clay and sand mixed, in varying proportions, 

 form Loam, or Brick-earth. 



Clay, with the addition of Lime, one-tenth or 

 more of its substance, becomes Marl, and in a dry 

 .state breaks up into cubical fragments. 



Marl, when indurated, becomes divisible into 

 laminae, and is termed Marl-slate. 



Note. " Bands of colour may sometimes be ob- 

 served on the sides of Slates, often coinciding with 

 slight changes of grain or texture ; these mark its 

 original stratification. But care must be taken in 

 field observations not to rely too implicitly on mere 

 bands of colour in slate rocks, unless they coincide 

 with bands of various texture, which may always be 

 trusted to show the original layers of deposition/'f 

 To ascertain the kind of any rock exposed in a quarry 



* Lyell's " Students' Elements/' p. 21. 



f Jukes' " Manual of Geology," p. 220. See also an excellent cut 



