chap.xiy. Porphyritic Clay stone Conglomerate. 473 



rounded, in others the angular,, fragments prevail, and 

 usually both kinds are mixed together : hence the word 

 breccia ought strictly to be appended to the term por- 

 phyritic conglomerate. The fragments consist of many 

 varieties of clay-stone porphyry, usually of nearly 

 the same colour with the surrounding basis, namely, 

 purplish-reddish, brownish, mottled or bright green; 

 occasionally fragments of a laminated, pale-coloured, 

 feldspathic rock, like altered clay-slate, are included ; 

 as are sometimes grains of quartz ; but only in one in- 

 stance in Central Chile (namely, at the mines of Jajuel) 

 a few pebbles of quartz. I nowhere observed mica in 

 this formation, and rarely hornblende ; where the latter 

 mineral did occur, I was generally in doubt whether 

 the mass really belonged to this formation, or was of 

 intrusive origin. Calcareous spar occasionally occurs 

 in small cavities ; and nests and layers of epidote are 

 common. In some few places in the finer-grained 

 varieties (for instance, at Quillota), there were short, 

 interrupted layers of earthy feldspar, which could be 

 traced, exactly as at Port Desire, passing into large 

 crystals of feldspar : I doubt, however, whether in this 

 instance the layers had ever been separately deposited 

 as tufaceous sediment. 



All the varieties of porphyritic conglomerates and 

 breccias pass into each other, and by innumerable 

 gradations into porphyries no longer retaining the least 

 trace of mechanical origin : the transition appears to 

 have been effected much more easily in the finer-grained, 

 than in the coarser-grained varieties. In one instance, 

 near Cauquenes, I noticed that a porphyritic conglo- 



pcbbles — for instance, flint pebbles from the chalk — are sometimes 

 zoned concentrically with their worn and rounded surfaces, might 

 have been led to infer that these balls of porphyry were not true 

 pebbles, but had originated in concretionary action. 



