186 
THE GEOLOGIST. 
Before leaving this part of my subject it may be well to glance at the 
frequency of wrong conclusions, arising from the presence of local 
deposits of lignite. In all clay beds slight deposits of carbonized 
wood may occur, Avhich, once observed by the unlearned, a coal-mining 
mania springs up — and thus the many futile attempts in the Liassic and 
Oolitic clays ; and it is probable that a famous opening in the Forest 
Marble clay, about two miles from Cirencester, was first attempted from 
the occurrence of this material, which is common in this rock*. Here, 
however, the good people fondly imagined that they had evidence more 
conclusive to rely upon than the famous Johanna Southcote, who 
prophesied that coal was there — no doubt, like the rest, having first 
examined the lignitic testimony. But, alas ! even prophecy cannot 
make coal ; and, after penetrating some distance into the Great Oolite, 
this work was abandoned. 
In considering the subject of where coal may be found convenient for 
working, I might refer to other special instances, but I prefer at present 
treating only of general principles connected with my subject, and will 
illustrate this part with an account of coal explorations undertaken by 
myself in America. 
The chief principles which should ever guide us in determining 
the convenience of working will be the following : — 
Depth s 
Physical circumstances ( q£ Coal. 
Quality and quantity ; 
The first of these is an important consideration ; as, if too deep to be 
worked with profit, there is usually an end to the speculation : besides, 
the probable depth is a matter to be ascertained, if possible, in 
estimating the cost before the work is commenced. As regards this 
item, it may be taken for granted that, as the shallow coal gets 
worked out, recourse will be had to the deeper beds. In the early days 
of mining, coal could only be won when near the surface ; but mines are 
«" Black, bituminous, and pyritous shales," (as well as lignite,) says Sir R. 
Murchison, " resembling beds of the coal formation, are quite enough to lead any 
common miner to believe that he " smells " the coal ; and thus country gentlemen 
are duped by ignorant men, who often honestly believe what they prophesy. 
Whether the strata thus resembling coal measures be a mile above or below the 
geological position of the carboniferous system has never formed part of the 
education of these speculators." — Silurian System, p. 488. 
