REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 113 



calcareous, and contain fossil wood enclosing crystals of calcite and analcime. Professor 

 Both, following Bunsen, 1 explains the presence of calcite by the decomposition into 

 palagonite of the volcanic materials associated with these fossil plants. Another layer 

 rests on this, formed also of palagonitic tufa, and containing fragments of fossil coniferous 

 wood. In the zeolitic basalt, forming a cliff toward the south of Christmas Harbour, two 

 beds of lignite occur at a height of 30 or 40 feet above sea level ; they are several feet 

 thick, and stretch towards Arch Bock. Silicified tree trunks were seen, according to 

 MacCormick, in the interior of this natural bridge. The lignite is schistoid, of a 

 brownish black colour, and varies much in composition. In some places it is earthy 

 and brittle, but in others it resembles the lignite of the Alps both in colour and 

 fracture. According to Captain von Schleinitz, quoted by Professor Both, quite similar 

 lignite is found in Breakwater Bay to the south of Cumberland Bay. 



To return now to the volcanic rocks of Christmas Harbour. From the position of the 

 Challenger's anchorage the naturalists could easily make themselves acquainted with 

 the disposition of the eruptive masses that border the bay. These form horizontal 

 layers and beds that may be followed along the whole extent of the vertical cliffs 

 which wall the fjord. Here, as in almost all the other parts of the island, the 

 eminences are terraces with flat summits. The plateau extending to north and south of 

 Christmas Harbour is broken by two mountains which rise above it ; to the north 

 there is Table Mountain, to the south a hill not yet possessed of a special name ; it 

 appears like an enormous block resting on the plateau. A part of these heights has 

 been named Mount Havergal, but it is evident that they are all formed of super- 

 imposed layers of basalt. The rocks rising above the horizontal beds of basalt and 

 forming the highest points of the series of mountains, are of phonolitic nature, 

 and similar to those which we shall describe in detail when speaking of Greenland 

 Harbour. They traverse the horizontal beds of basalt, from which they differ in 

 mineralogical character. Their eruption does not seem to have modified the arrange- 

 ment of the beds which surround them. The latter, forming the principal massif of 

 the region, are basaltic, and the beds are from 10 to 20 feet thick. These basalts are 

 massive, but by climbing the heights one comes to certain layers, the rocks of which 

 are vesicular and filled with zeolites (analcime and prismatic zeolites). These zeolitic 

 minerals are very common in this part of the island, where they are often found as 

 rounded grains in volcanic sand, with which their white colour affords a marked 

 contrast. From base to summit a regular alternation may be traced of beds of compact 

 sub-columnar basalt, and layers of the same material of a vesicular structure. These 

 amygdaloidal rocks appear in two chief forms : one has very small and numerous 

 vesicles, now completely filled with zeolites, the other has large cavities only lined by 



1 Ann. Chein. Ph., 1862, p. 53. 

 (PHYS. CHEM. CHALL. EXP. — FART VII. — 1889.) 1« 



