Geology and Magnetism. 493 



allow it to be more practical or useful than ' Fossil Geo- 

 logy,' a knowledge of which Mr. Hopkins supposes to be 

 of little consequence in mining operations, with regard 

 to Fossil Geology, and its practical applications to mining 

 operations. A good deal will, however, depend on the ob- 

 ject of those operations, whether they are directed for the 

 recovery of earthy, or of metallic minerals. Fossil Geology 

 would be of little use in directing the search for the precious 

 metals, which are generally found either in recent alluvium 

 or in the unstratified rocks in which no fossils occur. With 

 regard to coal, the case is very different, and in estimating 

 the value of indications of this mineral, we have no other rule 

 to guide us than fossil geology, and if this be not always 

 successful, the fault is attributable rather to its being imper- 

 fectly employed than to its being too much trusted to, and it 

 has the advantage over magnetic and economic geologies, in 

 as much as it is not to be counterfeited. 



After describing the Mariner's compass, its use and impor- 

 tance, Mr. Hopkins remarks, that there is no substance, but 

 which under suitable circumstances, is capable of exhibiting 

 signs of magnetic virtue. The crystalline rocks forming the 

 solid surface of the globe are more or less magnetic, and cause 

 great variations in the magnetic needle, which is never free 

 from such influence. Loadstone, a rock impregnated with 

 oxide of iron itself forms a magnet, and all primary crystal- 

 line rocks containing iron and manganese, will, with due 

 accuracy, point north and south like steel needles, i. e. in the 

 exact direction in which they are found in situ, when fresh 

 cut from the place in which they were formed. Even the 

 atmosphere is affected by the same influence, as when the 

 aurora borealis makes its appearance. The disturbance of 

 the magnetical equilibrium of the atmosphere occasions 

 what Humboldt terms magnetical storms, commonly observ- 

 ed it is said, before and during earthquakes in equatorial 

 America. 



3 s 



