474 Basal Strata of the Cordillera, paet n. 



merate assumed a spheroidal structure, and tended to 

 become columnar. Besides the porphyritic conglomerates 

 and the perfectly characterised porphyries of meta- 

 morphic origin, there are other porphyries which, 

 though differing not at all or only slightly in composi- 

 tion, certainly have had a different origin : these consist 

 of pink or purple clay-stone porphyries, sometimes 

 including grains of quartz, — of greenstone porphyry, 

 and of other dusky rocks, all generally porphyritic with 

 fine, large, tabular, opaque crystals, often placed cross- 

 wise, of feldspar cleaving like albite (judging from 

 several measurements), and often amygdaloidal with 

 silex, agate, carbonate of lime, green and brown bole. 1 

 These several porphyritic and amygdaloidal varieties 



1 This bole is a very common mineral in the amygdaloidal rocks ; 

 it is generally of a greenish-brown colour, with a radiating structure; 

 externally it is black with an almost metallic lustre, but often 

 coated by a bright green film. It is soft and can be scratched by a 

 quill ; under the blowpipe swells greatly and becomes scaly, then 

 fuses easily into a black magnetic bead. This substance is evidently 

 similar to that which often occurs in submarine volcanic rocks. 

 An examination of some very curious specimens of a fine porphyry 

 (from Jajuel) leads me to suspect that some of these amygdaloidal 

 balls, instead of having been deposited in pre-existing air-vesicles, 

 are of concretionary origin ; for in these specimens, some of the 

 pea-shaped little masses (often externally marked with minute pits) 

 are formed of a mixture of green earth with stony matter, like the 

 basis of theporphyry, including minute imperfect crystals of feldspar ; 

 and these pea-shaped little masses are themselves amygdaloidal with 

 minute spheres of the green earth, each enveloped by a film of white, 

 apparently feldspathic, earthy matter : so that theporphyry is doubly 

 amygdaloidal. It should not, however, be overlooked, that all the 

 strata here have undergone metamorphic action, which may have 

 caused crystals of feldspar to appear, and other changes to be 

 effected, in the originally simple amygdaloidal balls. Mr. J. D. 

 Dana, in an excellent paper on Trap rocks (' Edin. New Phil. Journ.' 

 vol. xli. p. 198), has argued, with gTeat force, that all amygdaloidal 

 minerals have been deposited by aqueous infiltration. I may take 

 this opportunity of alluding to a curious case, described in Chapter 

 II. of the present work, of an amygdaloidal rock, with many of the 

 cells, only half filled up with a mesotypic mineral. 



M. Rose has described an amygdaloid, brought by Dr. ITeyen 

 (' Reise urn Erde,' Th. 1, S. 316) from Chile, as consisting of crystall- 

 ised quartz, with crystals of stilbite within, and lined externally by 

 green earth. 



