132 ' Dr. Mac Culloch on a 



popular designations ; but that a regular gradation exists be- 

 tween all the varieties, from the driest Kilkenny coal or culm, 

 to the most bituminous coal of Newcastle ; or, chemically 

 speaking, from anthracite to asphaltum. 



The term commonly applied to this variety, by the miners, is 

 crystallized coal : that it is not a true crystallization, will be 

 apparent, though it will be difficult to refer it, as will presently 

 be seen, to any other class of forms or causes than that unde- 

 finable one, termed concretionary, and to the peculiar laws, 

 whatever those are, by which these forms, so often connected 

 with, or approaching to, perfect crystallizations, are regulated. 



It would have been desirable to give a finished and accurate 

 engraving of a form, which words alone will scarcely render in- 

 telligible 3 but as obvious causes render this impossible in your 

 Journal, your readers must be contented with an outline, which, 

 with that idea of the distribution of the general structure on the 

 surface, that can be conveyed in words, and the addition of a 

 few sentences, may perhaps render the subject sufficiently clear 

 for the present purpose. 



The surface of this variety of coal displays that involution of 

 lines, which is commonly known by the term snail-creep ; and 

 that character is accurately preserved in every specimen which 

 I have seen. These specimens, therefore, bear a general re- 

 semblance to that well known madreporite, vulgarly called brain- 

 stone : inasmuch that the first impression is, that this coal has 

 been in some manner connected with some organic body of this 

 nature. A moment's consideration, however, shows that this 

 could not have been the case ; and even a cursory examination 

 proves that, whatever the resemblance may be, the differences 

 are still greater. This apparent disposition is part of a structure 

 which perhaps the appended wood-cuts, though rather diagrams 

 than representations, may render intelligible. 



The outline in question is the protruding edge of a ridge 



