in the Neighbourhood of Edinburgh. 257 



while other fractured areas would be depressed ; so that, in this 

 manner, extensive tracts of surface would be raised out of water, 

 while other extensive tracts would be submerged. 



But these effects, resulting from thermometrical differences 

 communicated to the various fractured portions of the earth's 

 surface, would scarcely harmonize with oscillatory movements, un- 

 less we include in our theory an alternation of conditions possess- 

 ed at different times by such fractured portions. We must as- 

 sume, that the particular thermometrical condition by which one 

 fractured portion of the earth's crust was raised out of the water, 

 and another was depressed, might become, at different intervals, 

 liable to alternation ; otherwise we should fail in explaining why 

 the bed of a fresh-water lake should have so sunk beneath its 

 level, as to become covered with the waters of the sea ; — or why 

 the same submerged tract should have again risen from ocean's 

 depths, to resume its prior lacustrine condition ; — or why the 

 very fresh- water lake itself should have been liable to similar os- 

 cillations ; — why, for instance, a given portion of land, covered 

 with ferns or lycopodiaceae, should have subsided beneath the level 

 of lacustrine waters, or why it should have been again elevated 

 above them, and again have afforded the soil for a renewed 

 flora. 



An explanation has been thus attempted of the remarkable 

 alternations of fluviatile and marine strata, which are found in 

 the Lothian coal-district, and of the equally remarkable tranquil- 

 lity with which these alternations have been conducted. Mr De 

 la Beche, in reference to analogous phenomena which he has 

 observed in newer formations, conceives, that particular situations 

 are demanded to explain these deposits ; — situations where they 

 have not been exposed to the destructive action of waves or 

 powerful streams of water ; — and hence he argues, that fresh- 

 water lakes, like those of America, appear the localities which of- 

 fer the least difficult conditions. (De la Beche' s Theor. Geo- 

 logy, p. 162, 320, Sfc.J 



VOL. XIII. PART I. K k 



