ox THE so-called CBYBTALLIKE SflllSTS OF CHICHIBT-. 127 



If such be the case the compact original pyroxene, as it seems 

 to the writer, \v;is paramorphosed to the ba.<altic hornblende long 

 before the formation of the plane of parting in the former ; and by 

 that change the ferrous oxide r-ontained in the pyroxene became 

 ill ]:>art the ferric oxide, to which the lirown colour of the new 

 hornblende appears to be in the jnain attributable. When once the 

 feme oxide had been fjrmed, it ncted as a preventive to fiirtlier 

 chemical change in the substance of the hornblende, just as in the 

 case of an iron-rod which, when dipped into nitric acid becomes, by 

 virtue of the thin coating of oxide formed on its surface, passive or 

 indifferent to the acid. The process of oxidation stated above, had 

 advanced gradually from periphery of the pyroxene, while the inner 

 part of it remained quite intact; and at this stage mechanical forces 

 had acted from outside producing tlie lamellar, polysynthetic struc- 

 ture, very characteristic of diallage ; and at the same time lamella? 

 had been formed in the marginal, brown hornblende, though fewer 

 in number. Juddi)says; " If lamellar twinning has been already 

 developed in a crystal, then chemical action takes place along the 

 gliding planes in preference to the normal solution-planes." The 

 surface of lamella? in our diallaoe therefore atforded new fields for 

 chemical ])rocesses, and the mineral ultimately resolved itself into 

 tremolite or serpentine respectively, while the brown, compact horn- 

 blende survived the general decomposition. The above supposition 

 is moreover strongly supported by frequent occurrences of sharply 

 defined patches of the basaltic hornblende witliin tremolite and also 

 in a confused aggregate of serpentine. 



The felspar of the gabljro-diorite occurs in large allomorphic 

 crystals ; its appearance is quite dull, and is of a dirty grayish-white 



1) "Nature" Vo]. 35, Xo. IS, IVIarch, 18S7, p. 41(1. The Relation between Geology 

 anil Other Mineralocrical Si-icno;'«. — Presilentiat Address to the Geologial Society at the 

 Anniversary ^Nteeting, Fel)niary, 18S7. See also Quart. .Tourn. Geol. Soc. XLIII. 



