7B Dr. Mac Culloch on the Concretionary and 



Partly, perhaps, from the existence of amydgaloidal 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 Dr. Hutton 

 and Mr. Playfair, that these minerals also were the produce of 

 fusion, and that they had been secreted during the cooling of the 

 rock, so as, in fact, to form the cavities which they occupy. I 

 need not state here the various minute details, sometimes neither 

 very intelligible nor very requisite, by which this opinion was sup- 

 ported. The igneous theory of trap would be feeble indeed, had it 

 no firmer foundation than this to rest on; while the notion of a 

 chemical secretion is, to say the least of it, inconsistent with all 

 our chemical 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 consolida- 

 tion; but it is not to be explained how they should in this manner 

 form rounded nodules ; still less how the cavities that include 

 them should ever be partially empty, or present the peculiar sur? 

 face 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 that are entirely filled, 

 and to those that are utterly empty, it is a fair conclusion that the 

 whole alike owe their origin to mflation. It is then into previous 

 cavities that the minerals of the amygdaloids have been deposited ; 

 and it only remains to enquire whether this has been effected dur- 

 ing the igneous condition of the rock, or from posterior infiltrations 

 of a watery solution of earths. It must not here be objected that 

 the larger cavities could not have been produced by inflation ; 

 for as will presently be seen, it is in those, more particularly, 

 that the proofs of watery infiltration are more satisfactory. To 

 examine this question first. 



I have shown in the account of the Western Islands and else- 

 where, that stalactites of chalcedony were often found to depend 

 from the upper parts of such cavities partly filling the vacuity. In 

 other cases, the stalactite is found to correspond with an inferior 



