In certain types of basaltic lava,[[130]] when cooling and consolidation take place under peculiar conditions, such as we would expect to find in submarine eruptions, there is a residuum of the magma with relatively low fusibility that remains fluid after general solidification of the mass is well advanced. As the rock continues to consolidate, portions of this magma residuum become imprisoned in the mass, like whey in a cheese, giving rise to the “magma lakelets” above described; whilst other portions, during the contraction and fissuring accompanying the cooling process, are squeezed out into the cracks thus formed, or are intruded on the surface of the consolidating mass, as in the case of a submarine lava-flow. This solidified magma-residuum differs from the ordinary basic glass not only in its lower degree of fusibility but in its mineral composition and in its molecular condition. It probably in the first place does not differ much in appearance from the typical glass, but it is an unstable substance and is capable under certain hydro-chemical conditions of developing the characters of palagonite.
Showing fragments of glass with eroded borders and of plagioclase with more even edges in a matrix of palagonite traversed by cracks. The length of the largest fragment is half a millimetre. The glass has been evidently fractured in position and this is true of one of the felspar fragments. It is also apparent that whatever its cause the erosion of the margins of the glass has been produced since the fracture.
In those cases where the occurrence of palagonite is associated with evidence of crushing, the process appears to be in a sense reversed, since partial palagonitisation of an ordinary basic glass takes place as a result of the elevation of temperature due to the crushing. The heat thus developed is sufficient to partly fuse the glass; but since it is not great, it only affects the most fusible constituents, and the remelted material corresponds therefore to the magma-residuum of the consolidating mass, which is referred to in the previous paragraph. It has the same unstable characters and the same tendency to assume the palagonitic condition.
This theory centres around the relatively low fusibility of the magma-residuum that gives rise to palagonite. This degree of fusibility has yet to be ascertained, since according to the views here advanced it may even be much lower than that of tachylyte. It is, however, noteworthy that the melting-point of tachylyte is far below that of the more crystalline basaltic rocks, since it can be readily determined, as I have done in the instance of a dyke-like mass penetrated by tachylyte-veins before referred to, that the veins are composed of a much more fusible material than the rock-mass. From a very crude experiment I would infer that the melting-point of ordinary tachylyte is not much above that of lead (335° C). The fusion-point of an ordinary hemicrystalline basalt, according to the well-known experiments on the lavas of Vesuvius and Etna, would probably be over 1,000 degrees C.
Two interesting experiments, the one artificial, the other natural, may be here cited in connection with this view. Bunsen[[131]] more than half a century ago, as a result of some experiments in which he produced palagonite, arrived at the conclusion that the tuffs formed of this material are submarine deposits derived from the breaking up of previously formed palagonite-masses. Having obtained this substance by placing powdered basalt in an excess of melted potash-hydrate (Kalihydrat) and then adding water to the silicate of potash thus formed, he concluded that palagonite results from the reaction between glowing augitic-lavas and rocks rich in lime and other alkalies. Although Zirkel quotes in this connection the example where this material has been produced in the Cape de Verde Islands by basic lava flowing over limestone, he rejects Bunsen’s explanation as inapplicable to extensive palagonite districts, such as occur in Iceland, though allowing that it would account for the local production of this substance.
I venture to think, however, that in these two experiments the general principles involved in the production of palagonite are partly illustrated. We may accept the results of an experiment without acquiescing in its interpretation. As I take it, it is in the partial wet fusion of the powdered basalt that the secret of this successful production of palagonite lies. In both these experiments some of the conditions of a submarine flow have been reproduced.
Whilst Rosenbusch established the true character of palagonite as the product of a peculiar alteration of a basic glass, Renard pointed out the conditions under which it was most typically and in greatest abundance formed. But Bunsen was happy in his suggestion that palagonite-tuffs are submarine deposits derived from the breaking up of previously formed palagonite masses. The question thus resolves itself into one concerning the conditions of submarine eruptions and the behaviour during consolidation of a submarine basaltic flow. In the nature of things the field of investigation is mainly restricted to the examination of ancient submarine basaltic flows that have been raised above the sea.
A remarkable series of beds exposed in a stream-course below the Nandua tea-estate may be here described in connection with the question of the origin of palagonite formations. As observed on page [86], this locality lies on the flanks of a basaltic plateau, which are incrusted with recent submarine deposits. A pteropod-ooze, containing also the tests of large and small foraminifera and the shells of small bivalves, is displayed on the sides of the stream-course for the first 150 feet of the descent. Below this, as shown in the diagram, is a declivity with a drop of 60 or 70 feet where there is a waterfall. Horizontal beds of the pteropod-ooze rock are exposed in the upper-third of this declivity; but below, they pass into a chocolate-coloured marl-like deposit also horizontally bedded, and sometimes having a banded appearance from the alternation of layers of different degrees of fineness. This rock contains 5 or 6 per cent. of carbonate of lime and incloses a few scattered tests of minute foraminifera of the “Globigerina” and “Nodosaria” types. In the slide the rock appears to be of massive palagonite inclosing a few felspar-lathes ·1 to ·3 mm. in length, and exhibiting a zeolite and calcite in the crevices and cracks. But it was not until I had discovered the tests of the foraminifera and had observed some fragments of larger crystals of plagioclase and a little detritus of a semi-vitreous basaltic rock that its clastic character was disclosed. The palagonite change has here to a great extent disguised the character of the deposit.
This palagonite-marl formation is 20 or 30 feet in thickness. It passed downward into a reddish-brown rubbly unstratified rock which falls to pieces in one’s hands, breaking up into little cube-like masses an inch or two across. These masses display in their interior a radiate prismatic structure; but after drying they crumble into small fragments exhibiting the same minute prismatic structure, the miniature prisms being about a millimetre in diameter. Fine cracks, filled with calcite and a zeolite, traverse this rock in all directions, and no doubt this peculiar structure arises from shrinkage.