43 8 Patagonia. pabt n. 



aqueous origin is in many places shown both by the 

 arrangement of their smaller particles and by an oblique 

 or current lamination, also pass into porphyries, in 

 which every trace of mechanical origin and stratifica- 

 tion has been obliterated. 



Some highly porphyritic though coarse-grained 

 masses, evidently of sedimentary origin, and divided 

 into thin layers, differing from each other chiefly in 

 the number of embedded grains of quartz, interested 

 me much from the peculiar manner in which here and 

 there some of the layers terminated in abrupt points, 

 quite unlike those produced by a layer of sediment 

 naturally thinning out, and apparently the result of a 

 subsequent process of metamorphic aggregation. In 

 another common variety of a finer texture, the aggre- 

 gating process had gone further, for the whole mass 

 consisted of quite short, parallel, often slightly curved 

 layers or patches, of whitish or reddish finely granulo- 

 crystalline feldspathic matter, generally terminating at 

 both ends in blunt points ; these layers or patches fur- 

 ther tended to pass into wedge or almond shaped little 

 masses, and these finally into true crystals of feldspar, 

 with their centres often slightly drusy. The series was 

 so perfect that I could not doubt that these large 

 crystals, which had their longer axes placed parallel to 

 each other, had primarily originated in the metamor- 

 phosis and aggregation of alternating layers of tuff; 

 and hence their parallel position must be attributed 

 (unexpected though the conclusion may be), not to 

 laws of chemical action, but to the original planes of 

 deposition. I am tempted briefly to describe three 

 other singular allied varieties of rock; the first with- 

 out examination would have passed for a stratified 

 porphyritic breccia, but all the included angular frag- 

 ments consisted of a border of pinkish crystalline feld- 



