220 PREVAILING THEORY CONCERNING [Ch. XI. 



termed lava where it issues on the flanks of the mountain : 

 and we must therefore consider the terms trap and volcanic as 

 synonymous. 



Again, the difficulty of distinguishing the volcanic from 

 the Plutonic rocks is sufficiently great; for we must draw 

 an arbitrary line between them, there being an insensible 

 passage from the most common forms of granite into trap, as 

 detailed by Dr. Macculloch in his geological descriptions of 

 Aberdeenshire, and other parts of Scotland. It would be 

 easy to multiply examples to prove that the granitic and 

 trap-rocks pass into each other, and are merely different 

 forms which the same elements have assumed, according to 

 the different circumstances under which they have con- 

 solidated from a state of fusion. The same lava which is 

 porous where it has flowed over from the crater, and where 

 it has cooled rapidly and under comparatively slight pressure, 

 is compact and porphyritic in the dike. Now, these dikes 

 are evidently the channels of communication between the 

 crater and the volcanic foci below ; so that we may suppose 

 them to be continuous to the depth of several hundred 

 fathoms, or perhaps two or three miles, or even more ; and 

 the fluid matter below, which cools and consolidates under so 

 enormous a pressure, may be supposed to acquire a very dis- 

 tinct texture, and become granite. 



For these reasons, it is probable that Plutonic rocks have 

 originated in the nether parts of the earth's crust, as often 

 as the volcanic have been generated at the surface. And we 

 may also infer, that during each preceding period, whether 

 tertiary or secondary, there have been granites and gra- 

 nitiform rocks generated, because we have already discovered 

 the monuments of ancient volcanic eruptions at almost every 

 period. 



This supposition is corroborated by the observations of 

 several geologists, that granite is not confined to one par- 

 ticular period, antecedent to the introduction of organic 

 remains into our planet ; but has been produced again and 



