ON THE FLEXURES AND 



shores. It is so common that it would he superfluous 

 to quote examples ; hut it is interesting to point out 

 the kinds of rock in which it has been observed. It 

 is so frequent in the secondary sandstones, that it 

 must have fallen under the notice of every one ; and 

 it is often also found in the shales and limestones 

 which belong to the same series of rocks. The 

 cause appears to admit of no doubt ; as we are al- 

 ready sure, from the presence of marine shells in 

 the secondary strata, that they have been deposited 

 at the bottom of the sea. There is no difficulty in 

 understanding how those marks should remain in the 

 strata after consolidation ; as it will be found that 

 their planes are separated, exactly as those of the 

 level strata are, by changes in the quality of the ma- 

 terials, or by the interposition of another substance ; 

 just as the loose quartz sand on the shores suffers 

 mica or fine clay to be deposited on its surface. 



The very minute undulations which are sometimes 

 found in the argillaceous schists, differ from these 

 exceedingly, as well in their size as in their regula- 

 rity ; and if they have not arisen from the same cause, 

 it is to be feared that we are not very competent to 

 explain them. To say that they are modifications of 

 the concretionary structure, may perhaps be to ap- 

 proach the truth ; since we know of many appear- 

 ances far more extraordinary than this, which will 

 admit of no other explanation ; imperfect as that is, 

 from our total want of acquaintance with the true 

 nature of this mysterious process. 



The species of curvature next to be considered, is 

 of a very different nature, and opens a much wider 

 and more important field of inquiry. This sometimes 

 occurs on so large a scale as to involve whole districts, 



