(d) In submarine eruptions are to be found the conditions favouring the production of palagonite on a large scale. In the case of such basaltic flows it is probable that their upper portions are formed entirely of palagonite arising from the alteration of a vitreous magma-residuum extruded on the surface in the manner above described. Such a crust, as a result of shrinkage and other processes, would probably present itself to the geologist as a somewhat friable material, passing gradually into the overlying submarine deposits.
Note on the type of basalt found associated with palagonite.
The type is characterised, it would appear, rather by its structural features than by its mineral composition. It is the basalt of ophitic or semi-ophitic habit that would seem to be usually associated with palagonite; and since this habit is as a rule to be found where the groundmass displays large felspar-lathes in plexus arrangement, coarse augites, and at least a fair amount of smoky glass, it follows that a hemi-crystalline, ophitic or semi-ophitic, doleritic basalt is the type to be associated with palagonite.
This is the type of rock that forms the lower part of the basaltic flow near Kiombo, the upper part of which is largely palagonitic. To this structural type also belong most of the basalts in my collection where palagonite exists in the form of “magma-lakelets” in the groundmass. These “lakelets” are almost diagnostic of this type of basalt. Here also belongs the famous globular basalt of Acicastello on the coast of Sicily.[[132]] In such rocks the felspar-lathes form a mesh-work and vary usually in average length between ·1 and ·3 mm. The augites of the groundmass, typically semi-ophitic, range up to ·1 mm. in size. They are always large, that is, over ·03 mm., and this coarseness is another important indication.
Note on the changes produced through the hydration of palagonite.
Most of that which is detailed below is not according to my views palagonitisation, but the effect of hydration in the disintegration of this material. The initial molecular condition and the other characters which represent potentially the palagonitic change are not connected with hydration; but are concerned with the causes before explained that led to the formation of a basic glass of such an unstable constitution. Indeed, there is good reason to believe that the changes to be now described may be observed under the ordinary influences of weathering in a wet region.
The early stages of alteration are well displayed in some of the tuffs formed mainly of basic vacuolar glass, the submarine character of which is often indicated by a few tests of foraminifera. Whilst the glass retains its original bottle-green colour, it loses the clean sharp conchoidal edges and displays rough and uneven or granular borders. With a high power the surface of the fragment is seen to be minutely pitted or pock-marked in places, the shallow circular pits, less than ·01 mm. in diameter, being sometimes arranged in a row like a number of overlapping rain-prints. This process proceeds until all the surface is affected, and from this cause there is often an appearance of polygonal markings. The pock-marking, however, continues; and as the pits encroach more and more on each other an irregularly wrinkled rough surface results. Up to this time the glass has retained much of its original colour; but its clearness is replaced by turbidity, and collections of very minute rounded, rod-shaped, and irregular granules, composed of a colourless feebly polarising material, are displayed here and there in its substance, whilst some of the previously empty vacuoles are now filled with water.
In the next stage the hydration of the iron-oxides begins, and the glass becomes opaque and yellowish or reddish-brown, and has a more granular appearance, polarising feebly. Cracks now traverse the substance, and penetrate into the vacuoles, which, as they become filled with the alteration products, whether palagonitic, zeolitic, or siliceous, become ruptured and curiously distorted. The hydration and consequent disintegration continue until the deep stain of the iron-oxide is removed, and a semi-pulverulent whitish material remains. This is the history of the little bleached powdery patches so common in basic tuffs, each representing originally a lapillus of basic pumice. This powder when examined with the microscope is shown to be made up of fine granular and tubercular materials which lose much of their distinctness when mounted in Canada balsam. It is not affected by boiling in HCl, and contains usually an abundance of minute siliceous oval amygdules that have been freed in the last stage of the disintegration of the palagonite.
Such is the story of the degradation of the palagonite daily in operation in the basic tuffs of this island. From this source is doubtless derived much of the finest constituents of the submarine clays so common over Vanua Levu.