186 ON THE CONCRETIONARY AND 



forms. Lastly, similar cavities occur in the same 

 rocks, sometimes of considerable size, yet connected 

 by a gradation of magnitude with the smaller cells. 

 These seem to be the circumstances most essential to 

 the argument under review. 



Partly perhaps from the existence of amygdaloidal 

 nodules in volcanic rocks, and partly from a supposed 

 necessity for thinking that every mineral contained in 

 a trap rock must necessarily be, like its base, of 

 igneous origin, it has been argued by those who spe- 

 culated more than they observed, and reasoned ill from 

 what they saw, that these minerals also were the pro- 

 duce of fusion, and that they had been secreted 

 during the cooling of the rock, so as to form the 

 cavities which they occupy. I need not state the 

 various minute details, sometimes neither very intel- 

 ligible nor very requisite, by which this opinion was 

 supported. The igneous theory of trap would be 

 feeble indeed, had it no firmer foundation than this to 

 rest on ; while the notion of such a chemical secretion 

 is, to say the least of it, inconsistent with all our che- 

 inical experience. 



It is quite intelligible, that crystals of any mineral 

 should be formed in a fluid mass of the earths, as they 

 are in porphyries and in many volcanic products, 

 during the very process of consolidation ; but it is not 

 to be explained how they should in this manner form 

 rounded nodules ; still less, how the cavities which 

 include them should ever be partially empty, or 

 present the peculiar surface already described. The 

 vacant spaces must have contained an elastic fluid ; 

 and when we find that these vacancies are similar in 

 their forms and surfaces to the cavities which are 

 entirely filled, and to those which are utterly empty, it 

 is a fair conclusion that the whole, alike, owe their 



