Dr. Mac Culloch on the Lignites. 219 



form and structure justifying the name, either of coal or of lig- 

 nite, as it may happen, while there is an identity or resem- 

 blance in the chemical characters of both. 



The last geological situation in which the lignites occur, is 

 among the trap rocks ; and as they have either not been correctly 

 examined, or have been the objects of misapprehension, it is 

 necessary to be more particular in considering them. These 

 have been called, by some, bituminized wood, by others, basaltic 

 coal ; but it is more convenient to adopt the present term. 



They sometimes retain, in a considerable degree, the chemical 

 characters of vegetables ; giving out the particular volatile pro- 

 ducts on distillation, and the smell on burning, by which the other 

 lignites are distinguished ; while, at other times, they are in every 

 respect perfectly converted into coal. It is in the former case 

 chiefly that they retain the vegetable form and structure; but 

 they do not always lose it when they have become perfect coal. 

 Whether more or less distinct in character, they sometimes 

 occur in insulated fragments or as portions of trees ; at others, 

 they are accumulated in a particular place, so as to form small 

 deposites of an irregular nature, in which cases the vegetable form 

 disappears, or becomes very obscure. When such deposites are of 

 considerable size, they often put on the rocky character with the 

 chemical nature of coal, and are sometimes partially wrought for 

 economical uses. In one instance, which occurs in Mull, the 

 specimens of lignite form a large portion of a trunk of a tree ■ 

 in which the vegetable texture is perfectly distinct, but the 

 substance so tender, as to fall into powder by a very slight force, 

 resembling some of the specimens from Cologne. 



The circumstances under which the locks of the trap family 

 have been produced, render it necessary to state very accurately 

 the exact connexion subsisting between th*m and the lignites by 

 which they are accompanied. This is especially necessary, 

 because they have frequently been used as an argument against 

 the igneous origin of the trap rocks on the one hand ; whil£, by 

 another party, they have, under equal misapprehension, been 

 supposed to prove that origin, and fuither to justify the appli- 



