247 



hypothesis call attention to the similarity between the olivine of the 

 nodules and that of the basalts and to the absence of similar nodules in the 

 andesites and trachytes. BECKEK and SANDBERGER reply that the absence is 

 explained by the fact that the acid rocks have completely absorbed or 

 dissolved the nodules by means of the excess of silica contained in the more 

 acid magmas. In opposition to this it may be urged that perfectly angular 

 inclusions of basalt occur in acid rocks W and this being the case there is no 

 reason why inclusions of the ultra-basic rocks should not also occur. That 

 the rocks of the olivine-nodules have been developed under plutonic 

 conditions and not under the conditions that have given rise to the 

 formation of basalt will probably be admitted by all advocates of the 

 segregation hypothesis. Now we know that peridotites are constantly 

 found associated with gabbros and that both rocks are formed in connection 

 with the consolidation of basic magmas under plutonic conditions. This 

 fact tells strongly in favour of the hypothesis which regards the nodules 

 as being connected with the consolidation of the basic magma as against 

 the alternative hypothesis which regards them as fragments wholly foreign 

 to this magma. 



The absence of olivine-nodules from the Tertiary basalts and dolerites 

 of Great Britain, Ireland and Iceland is a striking feature and one which, 

 together with the general absence of porphyritic augite and the frequent 

 presence of the ophitic structure, seems to differentiate them from the 

 Continental basalts of the same age. The nearest allies of the Continental 

 basalts that we possess are some of the Carboniferous lavas of Scotland 

 and Ireland. These contain porphyritic augites and, in the case of the 

 rock from Limerick above referred to, nests of olivine-pseudomorphs which 

 suggest the former presence of olivine-nodules. 



The Limerick traps are often highly altered. One or more of the 

 minerals, serpentine, chlorite, calcite and epidote, are almost invariably 

 present. If we retain the term melaphyre for the altered basalts it may 

 be conveniently applied to such rocks as those now under consideration. 



The tuffs associated with the lavas are, as we should naturally expect, 

 still more highly altered. They abound in vesicular lapilli, the vesicles 

 now being filled with calcite. Fragments also of more compact micro- 

 porphyritic rocks are not uncommon. Calcite is present in great abundance 

 in the tuffs and may result in part from the decomposition of the original 

 silicates. The bulk of it, however, has very probably been introduced into 

 the rocks by waters percolating through the mountain limestone. 



At Kilala Bay in Co. Sligo an east and west dyke occurs in the 

 lowest members of the Carboniferous series. This is an extremely coarse 

 grained dolerite with ophitic structure. The ophitic augite is completely 

 subordinate to the felspar. It sometimes occurs as narrow plates between 

 broad lath-shaped felspars. The rock has been described by Professor 



VON LASAULX.< 2 > 



The augite is yellowish brown and slightly dichroic (a and y yellowish 



(1) See "Notes on the Rocks of St. Kilda," by A.Ross, British Ass. Report, 1885, p. 1040 

 (2, T. M. M. Neue Folge. Vol. I. (1878), p. 430. 



