Dr. Mac Culloch on the Lignites. 217 



The depth of the superincumbent strata, and the number of 

 alternations in these cases, present considerable variety ; but 

 observations have not as yet been sufficiently multiplied on this 

 subject, to admit of any general conclusions deserving of much 

 regard. The total depth at Bovey, is said to be seventy feet ; 

 this space including all the lignite, together with the alternating 

 beds of clay. In Iceland, it appears to be generally much more 

 superficial, and is found in the form of boards, as if produced 

 from the trunks of trees flattened by great pressure. This is the 

 variety to which the name of Surturbrand has been particularly 

 applied. That of Bardestrand is found on a hill of moderate 

 elevation, beneath strata of sand and clay alternating with peat ; 

 and here, as at Bovey, the upper beds are imperfect, the middle 

 intermediate in quality, and the lowest most complete. At 

 Arnafiord, it is accompanied by shale, containing bones, with 

 fragments of branches and roots. I need only add here that the 

 position of Jet is sometimes analogous. 



It thus appears that the species of lignite buried under alluvial 

 soils, occur in the several forms of brown coal, or common 

 woody lignite, pulverulent lignite or Cologne earth, surturbrand, 

 and jet. To whatever different circumstances these variations 

 may be owing, the differences, in a chemical view, are very con- 

 siderable, and imply either a longer exposure to the causes by 

 which the changes have been induced, or a much more energetic 

 action of these. 



It is difficult to form any conjecture respecting the date of 

 these deposites, as they regard the disposition of the present 

 surface of the earth. Inasmuch as they are superior to all the 

 rocks which they accompany, they may correspond to the general 

 or diluvian deposites of alluvia, while they may sometimes also 

 have been formed in estuaries at later periods. Even where 

 they have been produced by the consequences of volcanic erup- 

 tions, they must be of a comparatively recent origin. The first 

 species, if it exists, must be ranked in point of time with the 

 remains of extinct quadrupeds ; and thus, possibly, this is also 

 limited to particular situations, in consequence of circumstances 



