110 



such notions have arisen from the want of a more practical 

 knowledge of the general appearance and order of the disturbed 

 strata. Sometimes " faults " are filled with clay and various 

 other substances from the base below, or from the softer inter- 

 stratified seams by means of the pressure ; these clay veins are 

 impermeable, and consequently the water percolating in each 

 mass is retained by these natural dams, which is of great ad- 

 vantage to the independent workings of coal districts. The 

 coal on one side of a fault is often different from that on the 

 other side; sometimes losing its bituminous quality and be- 

 coming impregnated with other elements. Occasionally a per- 

 fect transition may be observed from anthracite to bituminous 

 coal, the modifying cause being from the former to the latter. 

 Not only is the water separated by the " faults," but also the 

 subterranean gases, the local accumulation of which is much 

 influenced by the angular position of the u faults " and dip of 

 the beds. This interesting question deserves a separate investi- 

 gation in connexion with the best mode of ventilation, or prac- 

 tical means of neutralizing the disastrous effects of these dan- 

 gerous elements, which the coal miner has to encounter : we 

 shall leave it to those who may have the opportunity of making 

 the necessary examination. 



CHAPTER XVI. 



GENERAL OBSERVATIONS. 



If we now take a general view of the effects of terrestrial mag- 

 netism in combination with other secondary agencies really ex- 

 isting, we find a sufficient natural cause to explain all the. cha- 

 racteristic changes which have been observed in the earth's con- 

 dition, in the degree, combination and sequence which actually 

 belong to them. Each of the phenomena, taken singly, is capa- 

 ble of demonstration in all its details of circumstances by the 

 operations of terrestrial magnetism in connexion with some 

 special branch of natural science. 



First. We have ample proofs of the existence of the magnetic 



