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THE BRITISH NATURALIST. [j, 



bodies ; and the same action is still going on at the present day, as is 

 clearly seen on comparing specimens of the Atlantic ooze, with those of 

 our chalk and limestone cliffs, their microscopical constituents bearing a 

 very striking resemblance. 



With regard to the " igneous " rocks, the term " igneous " is applied 

 to those rocks which have solidified from the molten state under the 

 earth's pressure, as opposed to the " volcanic " rocks which have been 

 ejected on the surface ; there are, nevertheless, intermediate stages, 

 which sometimes render it impossible to draw the line. 



The specimens in question, however, belong to the igneous group, the 

 outcome of a flood of molten rocks poured over the original bed of 

 limestone, which in due time was succeeded by a second layer, thus 

 covering the igneous rocks, and so accounting for their presence in the 

 midst of the limestone, the point of contact in the latter being marked 

 by an alteration in its composition, and the destruction of any crystal- 

 line structure present, owing to the baking and consolidation of the 

 molten rocks. Nor do the shales remain unaffected, for in those strata 

 lying on or under the toadstone, the calcined clay is burnt like a brick 

 to the depth of nearly a foot. 



In the process of cooling, steam and probably other gases have been 

 imprisoned in large quantities, forming innumerable small bubble-like 

 cavities in the upper part of the rock, many being now filled up with 

 calcareous sediment, etc. 



The rock itself is of a blackish brown colour, and extremely hard, 

 invariably calling into play the indispensable hammer, which the geologist 

 must wield with no light arm, if he intends to inflict any damage on 

 its, apparently, almost indestructible surface, offering, nevertheless, but 

 a vain resistance to the rain and wind, which latter fact brings us to the 

 consideration of the important relations these forces bear to the general 

 features of scenery. 



As we stand at the bottom of the dale, glancing up at the tall walls of 

 grey limestone, and then at the stream beneath, we are struck by the 

 peculiar angles at which it has carved out its course, some of them being 

 well-nigh acute ; and, we at once ask, is there no way of explaining the 

 phenomenon ? An explanation there is, depending upon two facts, 

 the importance of which cannot be overrated. 



(I) That water in which carbonic acid gas is dissolved becomes 

 invested with a new property, being able to dissolve up limestone. 



(II) That the limestone and, indeed, most other rocks have what are 

 technically called "joints," or certain lines of weakness in their structure, 

 such that any force like that of a river would first render its effects 

 visible along these lines. This is precisely what we meet with here : the 

 river having found out one of these lines, gradually excavates a course in 

 that direction, exerting nearly equal pressure on all sides till, finding 



