Desquamation of certain Rocks, 249 



pearanoes will induce geologists to turn their attention to the 

 actual investigation of that which they have hitherto taken for 

 granted without examination; and possibly enable them to 

 assign the true causes of those circumstances which the writer 

 of this paper professes himself unable to explain. 



I have here used the general term trap, merely to avoid cir- 

 cumlocution, as including, together with the greenstones and 

 basalts most commonly designated by this name, all the over- 

 lying mistratified rocks of whatever nature. It thus comprises 

 even the porphyries and syenites ; but the names of the several 

 species are also given wherever it has appeared necessary to 

 speak more definitely respecting them. 



The tendency to schistose, or flat laminar exfoliation on the 

 surfaces, is more common in the rocks of this extensive family, 

 than the spheroidal, since it occurs in every species that I have 

 examined, whereas the latter is rarely found except in basalts 

 and greenstones. An internal flat laminar structure, which 

 seems independent of the agency of the atmosphere, seems also 

 to exist in a greater number of species than the spheroidal con- 

 cretionary form does, and I shall first describe the cases in which 

 this modification occurs. 



I have as yet observed examples of a flat laminar structure 

 pervading a whole mass, only in veins and in prisms ; and, in 

 no instance, has a similar appearance occurred in amorphous 

 masses where it was not referable to the other cases of super- 

 ficial exfoliation, since it never penetrated deep into the mass. 

 It is not uncommon in the veins of porphyry with a base of com- 

 pact feldspar, which seem chiefly to exist in the primary strata; 

 and, in those, as it occupies the whole breadth of the vein, to 

 which it is moreover parallel, it appears independent of the 

 action of the atmosphere. It occurs also in veins of basalt and 

 of greenstone ; and, in some of these cases, veins of very con- 

 siderable breadth are found to put on this appearance through- 

 out. In all these cases, the thickness of the laminae varies from 

 the eighth to the third of an inch, or more ; and they are sepa- 

 rated from each other more or less perfectly, but always without 

 any signs of intermediate decomposition. Although no change 



