ASSOCIATED METAMOEPHIC EOCXS OP TnE LAKE-DISTEICT. 31 



degrees of fineness or coarseness of material, and many alternations 

 of various chemical composition. The changes produced have been 

 sometimes purely mechanical — such as the compression of ashy 

 particles into a very solid rock, or the elongation and fresh arrange- 

 ment of such particles produced in the process of cleavage by intense 

 lateral pressure. But, besides these, other and far greater changes 

 have taken place, variously in different beds, by that most powerful 

 of all agents, -water, acting in a more or less heated condition and 

 under pressure. The amount of alteration which has taken place 

 in the beds by atomic or molecular rearrangement, and the addition 

 or withdrawal of chemical elements, is most surprising ; and the mi- 

 croscopic study of the rocks brings this fact out in a most striking 

 manner. No trace of the original structure is left in many a rock 

 which outwardly presents no special signs of intense metamorphism. 

 Slice after slice may be cut and examined, and yet, perhaps, not a 

 single fragment of the most prominent mineral be found in a wholly 

 unaltered condition. And if this is the case among the less out- 

 wardly metamorphosed beds — if water at a comparatively low tem- 

 perature, though acting perhaps through vast periods of time, be 

 competent to produce such striking internal changes, what may not 

 result from that far more intensely acting metamorphism, of tho 

 same class, which has completely obliterated all signs of former ex- 

 ternal structure ? For it is a fact well worthy of careful notice, 

 that this original external structure (whether of bedding or frag- 

 mental) very long survives the destruction of internal structure ; and, 

 from the mere outside appearance of many a rock, none would have 

 the faintest idea of the enormous amount of change in its internal 

 and microscopic structure. 



Now, if we consider for a moment the action of metamorphism 

 upon a series of rocks formed of alternating beds of (1) moderately 

 fine-grained ash, (2) compact basaltic lava, (3) coarse brecciated 

 ash, (4) fine stratified volcanic dust of a general felspathic cha- 

 racter, (5) thin vesicular lava, (6) very fine-grained unstratified 

 volcanic dust, (7) volcanic breccia, we may form a fair idea of what 

 might be called selective metamorphism. The above-named deposits 

 might be thus changed into: — 1, a hard flinty-looking felstone 

 roughly cleaved ; 2, a rock with scarcely a trace of unaltered augite, 

 but full of chlorite, with quartz filling up spaces, and original crys- 

 talline structure almost or wholly obliterated ; 3, a ringing, white- 

 weathering felstone, perfectly structureless inside, but revealing the 

 outline of fragments on the exterior; 4, a felstone-like banded 

 rock, exceedingly trap-like within, and containing imperfectly 

 formed crystals of felspar ; 5, an amygdaloidal rock, charged through- 

 out with added material ; 6, apparently a fine-grained trap ; 7, a 

 massive rock, seeming to be composed of all sorts of metamorphic 

 rocks welded together. It is more than probable that in such a 

 series No. 6 would be mistaken for a bed of contemporaneous trap 

 (lava), of which 5 was the vesicular base, while 4 might be taken 

 to be a contemporaneous felstone trap. If these various beds were 

 of some thickness, and had an extended range, the errors supposed 



