chip. xiv. Porphyritic Clay stone Conglomerate. 477 



of between 2,000 and 3,000 feet, is thus composed. In 

 several parts of Northern Chile this formation extends 

 much farther towards the Pacific, over the granitic and 

 metamorphic lower rocks, than it does in Central Chile; 

 but the main Cordillera may be considered as its cen- 

 tral line, and its breadth in an east and west direction 

 is never great. At first the origin of this thick, mass- 

 ive, long but narrow formation, appeared to me very 

 anomalous : whence were derived, and how were dis- 

 persed the innumerable fragments, often of large size, 

 sometimes angular and sometimes rounded, and almost 

 invariably composed of porphyritic rocks ? Seeing that 

 the interstratified porphyries are never vesicular and 

 often not even amygdaloidal, we must conclude that the 

 pile was formed in deep water ; how then came so many 

 fragments to be well rounded and so many to remain 

 angular, sometimes the two kinds being equally mingled, 

 sometimes one and sometimes the other preponder- 

 ating ? That the clay-stone, greenstone, and other 

 porphyries and amygdaloids, which lie conformably be- 

 tween the beds of conglomerate, are ancient submarine 

 lavas, I think there can be no doubt ; and I believe we 

 must look to the craters whence these streams were 

 erupted, as the source of the breccia-conglomerate : 

 after a great explosion, we may fairly imagine that 

 the water in the heated and scarcely quiescent crater 

 would remain for a considerable time l sufficiently 

 agitated to triturate and round the loose fragments 

 lying within it : these rounded fragments, few or many 

 in number, would be shot forth at the next eruption, 

 associated with a few or many angular fragments, ac- 

 cording to the strength of the explosion. The porphyr- 



1 This certainly seems to have taken place in some recent volcanic 

 archipelagos, as at the Galapagos, where numerous craters are exclus- 

 ively formed of tuff and fragments of lava. 



