250 Dr. Mac CiiUoch oti the 



of texture is visible between approximate lamina?, this structure 

 seems to bear an analogy to that larger laminar structure not 

 unfrequent among these veins, in which the several laminaj 

 present different characters ; some being plain, others porphy- 

 ritic, and a third set amygdaloidal. As no action of the atmo- 

 sphere can be considered capable of producing the effect of a 

 laminar division, thus deep seated and parallel to the walls of a 

 vein, and on account of the analogy just noticed, there is reason 

 to conclude that all these cases are true examples of a concre- 

 tionary laminar structure. 



The most remarkable instance of a flat luminar structure 

 pervading a mass of prismatic form, occurs in Sky, in a green- 

 stone of fine texture; and, in this case, the division is parallel 

 to the axes of the prisms. It bears no resemblance therefore in 

 form to the spheroidal exfoliation of columns formerly noticed ; 

 as it does not take place at all on the transverse surfaces, 

 while it also occurs, in greater or less perfection, to such a 

 depth within the stone, as to be apparently free from the influ- 

 ence of external causes. It must also be remarked, that, both 

 in this instance and in that of the laminar veins, the integrity of 

 the structure is in no way affected, but that the laminae retain 

 even the original colour of the massive parts of the rock. 



It appears therefore reasonable to conclude, that, in this 

 case also, as in the former, the laminar division parallel to the 

 axes of the columns, depends primarily upon an original struc- 

 ture of that nature, which is developed, in all probability, by 

 some action of air or water, or both, analogous to that which, in 

 some of the cases formerly enumerated, seems capable of pro- 

 ducing it upon the exposed surfaces. 



1 must now enumerate those cases of a laminar exfoliation, 

 of much more frequent occurrence, the cause of which is far less 

 easy to assign, and which, although in some instances they may 

 be the consequences of a peculiar original structure, appear 

 often to be produced, as in some of the cases of granite and 

 others of spheroidal exfoliation in the trap rocks, by the mere 

 action of the atmosphere on the flat exposed surfaces. 



This phenomenon may be witnessed in so many parts of the 



