32 GEOLOGY OF THE LOTHIANS. 



curs, the explanation is difficult ; but when a plant is 

 found in a rock of mechanical formation, and completely 

 changed into that rock, the difficulty is increased ; and, in- 

 deed, in our present state of knowledge, concerning the 

 causes which effect the fossilization of organic bodies, it is, 

 perhaps, inexplicable. In other instances, however, the fos- 

 silized trunks which are found in the sandstone, are of 

 a composition entirely different from the sandstone, and 

 are changed, though more or less perfectininternal structure, 

 into a stony mass, all the characters of which indicate that 

 its solution has been entirely chemical. In the generality 

 of cases, there hasbeen a conversion into imperfect limestone, 

 which ishighly impregnated with carbonaceous matter; while 

 in others, there has been a change into a mass chiefly of a sili- 

 ceous nature. In all, the ligneous structure exhibits numerous 

 marks of derangement ; it is twisted, compressed, and other- 

 wise variously altered. In their animal relics, the strata of 

 the Lothians exhibit considerable variety, and have lately 

 afforded in their ichthyolitic remains much valuable know- 

 ledge. Like limestones having the same geognostical position 

 in other districts, those of the Lothians are distinguished by 

 the same characteristic fossils ; they abound in Product!, 

 and contain Orthoceratites, Nautilites, Euomphali, and 

 various species of Corallines, of which there are in some 

 places, as at Wardlaw in Linlithgowshire, and on the coast 

 near Aberlady, considerable beds entirely formed, one spe- 

 cies either constituting the whole bed, or two entering into 

 its composition. In the circumstance of the shells being 

 almost entirely confined to the limestone, there is an indica- 

 tion, that its formation is due to them, and their being pre- 

 served always entire, and not affording any marks of sub- 

 jection to attrition, prove that they lived and died in the 

 places where they are found : their absence, however, in the 

 sandstone, can never be considered at variance with the mine- 



