242 Dr. Mac Culloch on the 



by the rains or streams ; producing fragments and gravel, among 

 which imperfect and broken specimens are easily procured. It 

 is now necessary to add, that in these cases, the incipient de- 

 squamation cannot be completed by force. Neither, on breaking 

 through the whole block, where the partial division of the laminae 

 from it terminates, could it be suspected that the process would 

 continue further ; as no change whatever is perceptible at the 

 part where, without doubt, it is hereafter to be prolonged. 



In examining the detached laminae, the texture and appear- 

 ance seem in no way different from those of the solid mass 

 whence it was removed. Nor, as far as can be judged, is it less 

 tenacious than an artificial lamina of the same thickness would 

 be ; appearing indeed in every respect perfectly natural. Neither 

 does the surface exhibit any signs of decomposition, being on 

 the contrary brilliant and clean, as if cut by art ; even the feld- 

 spar scarcely assuming a perceptible degree of that opake white- 

 ness which this mineral so generally puts on after exposure to 

 air. The same cleanness and freshness of both the surfaces in 

 contact, are also found where the laminee and the block are se- 

 parated ; nor is there any loose matter generated, or any appear- 

 ances of decomposition in the plane which disjoins them. 



With respect to the thickness of the laminae, it was found to 

 vary from the eighth to the third of an inch, but it most com- 

 monly ranged from the sixth to the quarter. Although some- 

 times of a constant dimension throughout, they are observed to 

 be more frequently variable in thickness ; having a greater di- 

 mension at one side than another, or the surfaces being inclined. 

 The greatest superficial extent of the lamince which was ob- 

 served, did not exceed eighteen or twenty square inches, but, 

 from their brittleness and thickness, they can rarely be pro- 

 cured of half that size. 



It now remains to inquire, as already stated, whether this 

 process of desquamation is the result of an internal schistose 

 structure invisible in the solid block, as it is in those cases where 

 it is spheroidal from different centres, and only called into 

 action by exposure to the atmosphere, or whether, as in the case 



