TRUE VOLCANOES. 451 



tures, but from a net-work of fissures on the surface of the 

 earth frequently covering over in the form of strata a space 

 of many square miles. To these probably belong those masses 

 of trap of the lower Silurian formation of the southwest of 

 England, by the chronometric determination of which my 

 worthy friend, Sir Roderic Murchison, has so greatly increased 

 and heightened our acquaintance with the geological con- 

 struction of the globe. 



