366 ON VOLCANOES AND EARTHQUAKES. 



have been deduced groundless conclusions respecting 

 the ages of eruptions, which it is now no longer ne- 

 cessary to refute. 



It has heen idly disputed whether lavas could ever 

 he compact: and hence they who have denied this 

 possibility, have referred all the compact volcanic 

 rocks to trap. This has heen a fertile source of er- 

 rors which it is important to remove; both as it re- 

 gards the history of the former and that of the latter 

 substances. The question can never be decided, it is 

 evident, if every compact lava is considered a trap rock; 

 since this is to assume the point to be proved. The 

 volcanic rocks of Auvergne are often compact, as are 

 many deep currents of lava which have been examined 

 in many parts of the world. And as, in many modern 

 lavas, the size of the pores diminishes as we descend 

 deeper in the bed, the possibility of this state of abso- 

 lute solidity, and the nature of the cause by which the 

 variable nature of lava in this respect is influenced are 

 proved. It is elsewhere shown that the same variations 

 exist in the trap rocks; and unquestionably from the 

 same cause, or a greater or less degree of pressure acting 

 on the fluid material. The analogy of lavas, in many 

 particulars, to the rocks of this class, is striking; as 

 must be the case, since their origin is analogous or 

 identical. Much additional light would be thrown 

 on the latter, from a more accurate history of vol- 

 canoes than we have yet had. It is an important 

 point to ascertain, whether the rocks, apparently of 

 the trap family, in which volcanoes, or lavas, are often 

 situated, are the produce of their eruptions, and, 

 further, whether they are the produce of eruptions 

 within recent times, or during the present state of the 

 globe. If such formations of trap can be certainly 

 distinguished from the ascertained produce of recent 



