226 Dr. Mac Culloch on the Hill of KinnouL 



appearing to be the stalagmite formed on the bottom of the cavity, 

 and proceeding from below upwards in successive additions, in a 

 way similar to that which may be observed in the complicated 

 chalcedonies of Faroe. The peculiarity observable in the fourth % 

 example can be explained only by a similar supposition. 



Such are the circumstances of chief importance which occur in 

 the trap nodules, and which, although by no means limited to this 

 place, appear deserving of notice, on account of the intimate 

 relation they bear to any general theory. It is incumbent on that 

 hypothesis which explains one of these difficulties, to explain the 

 others also, or at least to require nothing which shall exclude the 

 means of explaining them. It is almost needless to say that I here 

 allude to the different explanations which the aqueous and the 

 igneous theories of the origin of trap have given of the imbedded 

 nodules. Each hypothesis has its difficulties when it refuses to 

 yield ground to its antagonist ; yet, is each perhaps incapable of 

 exclusively and clearly explaining the appearances in question. 

 The stalactitical forms which line the hollow cavities, can perhaps 

 have resulted only from watery infiltration, yet the existence of 

 the previous cavities can scarcely be accounted for by that hypothe- 

 sis which considers trap as a deposit crystallized from an aqueous 

 solution of earths. 



Different substances are found occupying veins as well as nodules 

 in this rock ; among these, calcareous spar and quartz are the most 

 common. Sulphate of barytes occurs more rarely, as do chert and 

 agate, the latter of which often exhibits the same zoned appear- 

 ance in the veins which it does in the nodules. 



The great abundance of chlorite in this trap might have rendered 

 it probable that the rare mineral heliotrope was an inmate of it: 



\ PI. 10. N° 4. 



