APPENDIX. 167 



Residue : 59.6 per cent, dried at 110° 0. 



Ignition loss of same : 8.32 per cent. 



Extract, in percentages of original material : — • 



SiOj 6.27 



AI3O3 11.07 



Fe,Oa 1.57 



CaO ... • 0.44 



MgO 1.65 



K2O 3.37 



Na^O . 2.70 



27.07 



The substance is thus divided into : — 



Total water 18.36 per cent. 



Total extract 27.07 " " 



Anhydrous residue 54.60 " " 



Prom this experiment it is seen that the substance gives up about three-fourths of its 

 bases, and a larger proportion than this of its alkalies, when boiled with quite dilute acid. 



Altogether, then, it may be concluded that we have at this station a thick semi-compact 

 deposit of fairly pure zeolitic matter. 



Loose concretions of white matter similar to this occur at Stations 4695 (2020 fathoms) 

 and 4736 (2289 fathoms) in large slabs, and in the nuclei of manganese nodules at Station 

 2. It is also found in the manganese nodules of several of the " Challenger " stations. The 

 tuff from Station 4695 is comparatively ferruginous, and has an ochreous appearance, 

 though parts of it are just as white as that from Station 2. Both this tuff, and still more 

 markedly that from Station 4736, contained much decomposed volcanic glass, which either 

 remains as honey-colored palagonite or has disappeared, leaving vacuoles lined with limo- 

 nite. In this respect the last named tuff resembles the white nuclei of nodules from " Chal- 

 lenger" Stations 285 and 297, with which substance, indeed, it appears to be identical. 



As regards the origin of palagonitic tuffs such as those enumerated, it seems clear from 

 the accompanying minerals and general associations that they arise from the decomposition 

 of layers of volcanic ash which have fallen on a Red Clay bottom. Assuming the white 

 matter to consist of zeolites, we have two possible mother-substances for it, viz., basic vol- 

 canic glass and felspar. These always occur together in volcanic ash, and the generation 

 of zeolites from the one and the other under submarine decomposition has been amply 

 demonstrated by the mineralogical examination of the " Challenger " dredgings. The zeo- 

 lites have been deposited from solution, as is evidenced by their proneness to form casts in 

 Globigerina shells,- and have often the appearance of having soaked, as it were, out of the 

 mother-substance. 



Now the peculiarity of the tuff from Station 2 lies in the almost complete absence of 

 true palagonite or the remains of palagonite, whence its white color and low iron-content. 

 The layer of rock-silicates from which it was derived would seem therefore to have con- 

 sisted mainly of acid minerals with an insignificant proportion of ferro-magnesian ores ; 

 and since there is no evidence of the presence of rhyolitic glass in the deposit or neighbor- 

 hood, the minerals were, doubtless, chiefly felspathic. That is, the original volcanic ash 

 must have been more or less sorted before reaching the bottom, presumably by reason of 

 the lower specific gravity of felspar. We may suppose either that a richly ferruginous 

 layer of ash sank home underneath a felspathic layer, or, what is more probable, that the 

 felspathic fragments, whilst sinking, were carried sideways by an oceanic current, and 



