ACADEMY OF NATURAL SCIENCES. 395 



rock to the surrounding undecomposed country, though sometimes rapid is 

 always gradual, so far as I have seen ; and though we cannot yet speak much 

 from underground explorations, the surface appearances throughout the country 

 would indicate decidedly that so regular a line of demarcation as this at Quail 

 Hill is the exception, and not the rule. The southwestern limit of the decom- 

 posed mass of Quail Hill has been found at several points ; but here the change 

 from the decomposed to the undecomposed rock is not so rapid ; and though 

 the explorations here, being shallow and limited, are insufficient to determine 

 this point with certainty, it is not probable that any such regularity of demarc- 

 ation exists here as upon the opposite side. Most of the " calico rock "' of this 

 belt still retains distinctly the structure of the undecomposed rock from which 

 it was formed. The crystalline hornblendic rock is thus seen to have been 

 largely altered by the decomposing agency, and even the hornstone, which lay 

 in its track, seems to have been more or less affected by it. The decomposition 

 has been purely an oxidation, accompanied by such mechanical and chemical 

 changes as filtering mineral waters might produce. It is probably superficial, 

 both in origin and character, extending to no great depth, although the main 

 level at Quail Hill is nearly one hundred and twenty feet beneath the summit of 

 the hill, and the decomposition of most of the rock at this depth, so far as 

 exploration has gone, is as perfect as at any higher level. It is certainly long 

 subsequent in date to the metamorphism of the surrounding country, and is 

 unquestionably largely due to the action of the products of the oxidation of 

 metallic sulphurets (chiefly those of iron and copper) which were originally 

 distributed through the rock. At the same time it is not easy to account for 

 the whole of it in this way alone, since at certain localities undecomposed sul- 

 phurets are seen near the surface, and in rock which is apparently much more 

 permeable to atmospheric influences than was much of that which has been 

 more deeply decomposed ; and again, much of the decomposed ropk, though 

 retaining well its original structure, shows far too little traces of sulphurets to 

 readily account for so general and thorough a decomposition as has taken place. 

 It is all indeed more or less colored by oxide of iron, but much of it is not 

 deeply colored, and the undecomposed hornblendic rock itself, in the absence of 

 all sulphurets, contains sufficient iron in the state of protoxide to impart a strong 

 coloring when the rock is decomposed and the iron passes to the state of sesqui- 

 oxide. Much of the iron originally present has undoubtedly been removed 

 in a soluble form, as sulphate, etc. But in rock which preserves its original 

 structure, as well as most of this does, pyrites, if originally present, would have 

 left traces of its existence in the form of casts or cavities in the deomposed 

 mass, which might or might not have been filled with ferric oxide or other 

 matter. In certain localities the decomposed rock is in fact filled with such 

 cavities, often cubical in form, attesting the former presence of large quantities 

 of disseminated sulphurets. But in other localities they are few and far between, 

 and here accordingly the decomposition can hardly be supposed to have been 

 due to the local presence of sulphurets alone. 



The exact methods by which the general and local decomposition has been 

 effected, and those by which the rock was originally impregnated with metallic 



