On Underground Temperatures, 



4<> 



Mean Mean 

 conductivity. resistance. 

 k. r. 



-00433 .... 267 



. . -00473 .... 225 



.. 0-00308 .... 331 



1. Coal Mines (carboniferous strata). 



Sandstones ") 



Shales ; Clays ] " 



2. Mineral Mines (metamorphic and 



crystalline rocks). 



Crystalline rocks 'I 



Schistose rocks ; Clay-slates. . . J " 



3. Artesian Wells (mesozoic and ter- 



tiary strata). 

 Soft, and New Red, sandstones 



Oolites 



Chalk ; Greensands 



Clays ; Marlstones (Lias) .... 



This shows a considerable difference between the conductivity of the 

 metamorphic and palaeozoic rocks of the Mineral and Coal Mines, and 

 that of the newer strata in which the Artesian Wells are usually 

 situated ; but there are conditions, hydro-geological and structural, 

 obtaining in the rock masses themselves, which introduce many modi- 

 fications affecting the value of these differences. 



For example, in each group there are subordinate beds, which may 

 have more or less local influence. In the first there are seams of coal, 

 of which the conductivity is extremely small, though these seams form 

 but a fraction of the entire mass. Thus there are — 



Total 



No. of Total thickness 



workable thickness of coal- 



seams, of coal. measures. 



16 .... 46ft 3,000ft: 



30 .... 130 „ ... . 5,000 „ 

 75 .... 126 „ ... . 11,000 „ 



In the coalfield of Newcastle 



„ „ N. Staffordshire 



South Wales . . 



The coal therefore only enters as a very small fractional part into 

 the constitution of the Coal-measures. Besides, the coal seams vary 

 very much in thickness, and are of limited range, that is to say, they 

 form thin plates never coextensive with the Coal-measures them- 

 selves ; at the same time, when one thins out, it may be replaced by 

 another on a different level. 



In the crystalline and schistose rocks, veins and layers of quartz 

 and of quartzite are of common occurrence, though very irregular in 

 their mode of distribution. " The quartz veins and seams are generally 

 thin, but the quartzite often forms masses of large dimensions. 



In the Artesian Wells gypsum and rock salt are of not unfrequent 

 occurrence in the Triassic strata ; in the remarkable instance of the 



vol. xli. e 



