o* OUTLINES OF FIELD-GEOLOGY PARTI 



of chalk. A stiff argillaceous soil, abound ir 

 smoothed stones, many of them well - striated, would 

 prove that a boulder-clay or till lay hi 1 \\ A profusion 

 of fragments of some peculiar rock, a basalt, for example, 

 or a diorite, or a porphyrite, extending in a definite band 

 across a field or hillside, would probably show us that a 

 rock of that character existed, in situ, somewhere in the 

 immediate neighbourhood of the fragments. We require, 

 of course, in all these cases, to go carefully over the 

 ground, and draw our conclusion only after we have 

 exhausted all the evidence procurable. 



But it may be remarked that, except on freshly- 

 ploughed land, the soil is not bare and exposed to our 

 scrutiny ; that, on the contrary, it is commonly just as 

 much concealed by its coating of vegetation as the hard 

 rocks are by their covering of soil. Even under the 

 most unfavourable circumstances, however, the geologist 

 may glean not a little of the information which he needs. 

 Where the ground slopes, he will probably have no great 

 trouble in finding some little rut or trench which has 

 been cut, or at least deepened, by rain, and where he 

 will obtain access to the underlying soil, or even, it may 

 be, to the subsoil and the still undecomposed rock below 

 it. Where, on the other .hand, the ground is too flat to 

 hope for assistance from rain -action, he will look for 

 traces of burrowing animals, by which the soil may have 

 been thrown up to the surface. In Britain, the common 

 earth-worm, the mole, and the rabbit, are excellent co- 

 adjutors in his work. The fine castings of the earthworm 

 give him at least the colour and general constitution of 

 the soil, whether sandy or clayey. The heaps of the 



