VOLCANIC ROCKS. 596 



probable, however, that another set of difficul- 

 ties will always exist : they may even be increased ; 

 since, if the trap rocks are of a local or partial 

 nature, and the produce of actions similar to those 

 now present in existing volcanoes, it is by no means 

 improbable that they may occupy the same places, 

 as some of them actually appear to do, and that 

 the produce of the distant and the recent opera- 

 tions of fire, will thus be confounded. But, on 

 this interesting part of geology, it is not here pos- 

 sible to proceed to any further discussion. 



i 



From the imperfect and often prejudiced de- 

 scriptions of writers, it is therefore impossible to 

 know under what different forms the volcanic 

 rocks actually exist. But those are, on all hands, 

 admitted to belong to this division, which, how- 

 ever antient, are connected with existing or ex- 

 tinct volcanoes in the form assumed by currents 

 of modern lava. 



The extent of these rocks is extremely vari- 

 able, as are, necessarily, their forms and disposi- 

 tion ; these circumstances depending on the form 

 of the ground over which they have flowed, and 

 no the quantity and the nature of the materials. 

 Q Q 2 



