318 SKY. GEOLOGY. SECONDARY STRATA. 



in the sandstone, we may perhaps safely conclude that 

 the lowest beds are, in a general sense, parallel to this 

 rock. The contact of the two may however be observed 

 in a few detached points ; in others, as near Lucy, where 

 it might be expected, there is a very near approximation 

 in conformable order ; but the important interval, as most 

 commonly happens, is filled with rubbish. This is a 

 circumstance of frequent occurrence at the meeting of 

 approximate rocks, and is a common cause of disappoint- 

 ment to the geologist. It seems to arise from a decom- 

 position of the exposed parts of the substances at their 

 junction; and occurs nearly as often at the contact of 

 two stratified rocks as it does where these are traversed 

 by the unstratified. Hence, from the wearing of the 

 materials at the surface, a cavity is produced ; which 

 becoming subsequently filled with rubbish and generally 

 covered with a vegetable soil of unusual depth, effectually 

 prevents a view of the contiguous parts. It must how- 

 ever still be remarked respecting the common boundary 

 of the sandstone and limestone, that although the course 

 of the former is rectilinear within itself, and a similar 

 straight tendency is to be seen in the latter wherever 

 they can be traced, yet that straight line is not preserved 

 at their junction; as the map will show that they are 

 divided by a line which does not coincide with that of 

 their bearings. It is easy to conceive, that, notwithstand- 

 ing a perfect geological conformity between the two, 

 such a disposition may result from the uneven state of 

 the surface, or from variations in the dip. It may also 

 happen that, with a similar general conformity of bearing, 

 the limestone may occupy a basin-shaped cavity, and thus 

 possess an uneven boundary ; nor is it improbable that 

 such is the condition of the strata in question. The rea- 

 sons which render that supposition probable will manifest 

 themselves hereafter, when the islands that adjoin Sky 

 and resemble it in composition have been described. 



