424 Dr. Mac Culloch on the Geology of 



may perceive that it is of a large lamellar form, or such that one of 

 the lateral dimensions is always much greater than the other, the 

 leading lines being nearly vertical. The cross fracture by which it 

 Is separated into small masses and becomes detached, is compara- 

 tively so rare as not to give any general feature. If it is possible 

 to transfer a term from a hand specimen to a rock, I might say that 

 it was formed of distinct lamellar concretions, and a perfect idea 

 may be acquired of it by conceiving the lamellar pitchstone of 

 Arran magnified to the requisite size. It contains some caverns, 

 formed in a vertical direction, but whether by a falling cut of some 

 parts, or a subsidence and consequent separation of contiguous ones 

 I could not determine. I have already said that its texture and 

 aspect are very various, its varieties occurring as far as I could per- 

 ceive without any certain order or regularity. Its basis appears to 

 be a felspar, occasionally of the compact sort, but more generally 

 crystallized in a confused manner. It differs much in colour, and 

 offers the following remarkable varieties, brown red, umber brown, 

 iron grey, purple brown, ash colour, yellow green, olive green, 

 pale grey green, and grass green. The crystals of felspar which 

 are imbedded in the mass are of a small size, rarely exceeding the 

 tenth of an inch, and are either grass green, brown, or white. 

 Nodules of green earth are also found imbedded in it, and the 

 green varieties appear to derive their colour from this substance. 



I have contented myself according to the established language 

 with calling this rock by the simple name of porphyry, a name so 

 vague, and so carelessly applied to rocks of widely different charac- 

 t-ers, as to be a cause of unspeakable confusion. Since the term por- 

 phyry is now applied to all rocks, whatever they may be, which in 

 a given base contain imbedded crystals of felspar, or of any other 

 substance, it is evident that it can determine nothing with regard to 



