138 THE VOYAGE OF H.M.S. CHALLENGER. 



taken by delessite with the addition of chalcedony, as is often seen in the volcanic 

 products of Kerguelen. 



Without knowing the stratigraphic relations of the rocks of Foul House Bay, a 

 summary of the lithological characters of which has just been given, they might be 

 equally well classed as peridotic diabases or recent dolerites, but the probabilities are 

 in favour of the latter supposition. 



Taking a summary view of the general observations given in the preceding pages, 

 and those made at Kerguelen by the various naturalists who have explored the island, 

 we see that the physical geography, the disposition, and the nature of the rocks all show 

 the island to be of volcanic origin, and that the eruptive masses of basalt and trachyte 

 belong to recent periods. The basalt formerly spread in vast continuous sheets far 

 beyond the present limits of the land. The oscillations of the land, the erosive action 

 of the atmosphere, of glaciers, and of the waves, have eaten into and carved out the 

 coasts of Kerguelen, thus giving it its actual relief and remarkable outlines. 



If we take into account all the observations of British and German naturalists, par- 

 ticularly those of Dr. Studer, it must be admitted that Kerguelen Island has been, in the 

 main, built up by successive eruptions of basaltic masses spread out in wide outflows. At 

 some points as many as twenty of these sheets can be counted one above another. All 

 these basaltic rocks are felspathic, and are associated in a subsidiary way with palagonitic 

 tufas and limburgite ; they present great uniformity in structure and composition in all 

 parts of the island. Dolerites appear to predominate, and amygdaloidal rocks with 

 zeolites and geodes of quartz and chalcedony are very common amongst them. All the 

 rocks of this series are connected together by their composition, and the different modes 

 of structure they present may easily be explained. In fact, it is observed that the 

 numerous basalt sheets are fine grained at the bottom and centre, but alveolar or even 

 scoriaceous in the upper part, i.e., the original surface of the stream. This surface is 

 in its turn covered by a more massive rock. It must be admitted that, as in the case 

 of lava - streams, the scoriaceous or amygdaloidal portion corresponds to the upper 

 surface of the lava. Here the expansion of imprisoned gases was not counterbalanced 

 by the pressure of the overlaying rocks, as was the case in the lower parts of the 

 bed. The eruptions have been subaerial, at least in most cases. These facts, so far 

 as they are exhibited in the neighbourhood of the German station, were observed in 

 detail by Dr. Studer, and may be generalised for all other parts of the island ; they are 

 shown well at Christmas Harbour. 



The terraced structure of these volcanic hills is due to the manner in which the 

 masses composing them were erupted. One might suppose that the successive outflows 

 were superimposed on beds of a former eruption without covering their whole surface, 



