292 



O/) the Criista/liiii' Stn/cfurc of the 



[A ntif. 



most commonly met with— Clialk in the vicitiity of basall, is known to 

 become beautifully crystalline : the crystallization being most perfect, in 

 the immeiliate neighbourhood of the altering rock, while it becomes 

 gradually less and less distinct as we recede from this. Numberless in- 

 stances similar, ami even more striking, than the above might be ad- 

 duced, the facts however are so familiar to every geologist, that I tlo 

 not consider it necessary to advance more ; in all, however, this is ob- 

 served, that crystals only appear in those localities, atFected by the 

 dyke. The sienite, including the trap dykes of Amboor, exhibited no 

 signs of alteration; but this is by no means aii uncommon circumstance, 

 and it tinds a ready explanation, when we bear in mind, that the origi- 

 nal temperature of injected rocks may vary considerably, and also that 

 the conducting powers of different rocks, on which their susceptibility 

 of alteration materially depends, are also very variable ; from these 

 two causes, it may frequently happen, that certain rocks exhibit no 

 signs of alteration, under circumstances apparently similar to those of 

 others, in which these signs are strikingly apparent.* 



The three preceding classes belong peculiarly to the province of the 

 geologist, all being taken from his particular science. Now in this, it 

 most frequently haj)pens, that eflects only remain tons, the causes to 

 which these are due liaving ceased to act ; it therefore becomes necessa- 

 ry to advance with caution, in our enquiry as to what these causes 

 may have been, and rigidly to examine and verify each step we take. 

 Hence, then, it is very natural for us to enquire, if there is a possibility 

 of producing efiTects analogous to those observed in the natural world, 

 while we can watch also the cause in operation to produce .them. This 

 remark leads us to the consideration of a fourth class, by which some 

 additional light is thrown on the enquiry. 



(4). In this class, I purpose including the results of the researches 

 of several eminent chemists, on the production of crystals, similar to 

 those occurring in nature, through the agency of heat, of greater or less 

 intensity. Among these, Mitscherlich of Berlin has been the most suc- 

 cessful, and to him we are indebted for a large proportion of the new 

 and interesting information these researches have furnished. Having 

 by careful analysis, made himself acquainted with the proportions, ex- 

 isting between the component parts of different minerals, he formed a 

 mixture, in which these were retained, and after exposing this to the 



* On examinind; the sienite of Wan i umbo tidy iu which trap dykes are also abundantly 

 met with, I was equally unsuccessful in ray search after signs of alteration. Benza 

 howevtfr mentions having met with thorn occasionally. In Antrim, Isle of Anglesea^ 

 these alteration'^ are remarliable. 



