AND GEAVELS OE AEDTEN, ETC., IX MELL. 



277 



of a more calcareous nature when first included in the gravels *. In 

 section they afford a rich harvest of organic remains, including, in 

 one of the instances examined, glauconitic casts sufficiently nume- 

 rous to give a distinct greenish tinge to the cut surface of the 

 specimen. 



Pebbles of grey quartzite are also found, and the matrix of the 

 gravel is composed of quartz- and felspar-sand, comminuted parti- 

 cles of lava, and occasional glauconitic grains. Though the crystal- 

 line granules may have been largely derived from the old gneissic 

 floor of the district, yet in some cases they retain intrusions of glassy 

 matter which indicate their original occurrence as porphyritic crys- 

 tals in dykes or lava-flows. 



The pebbles of volcanic material might be expected to exhibit 

 many interesting characters, such as are usually lost to us by wea- 

 thering before a lava-stream becomes entombed among later accumu- 

 lations f . We find, in fact, rolled fragments of scoriaceous basalt- 

 surfaces ; and the microscope reveals many other products of rapid 

 cooling, such as colourless pumice and particles of basic glass, the 

 latter showing fluidai structure and crystallites in various stages 

 of development. 



A large number of the pebbles, whether grey-green, brown, or 

 even pink, are derived from pre-existing basaltic flows, which have 

 yielded specimens of their more compact, though not of their doleritic, 

 portions. In one brown-pink example we have, with a fresh mono- 

 clinic pyroxene, olivine so ready in its decomposition, and giving rise 

 to such rich brown products, as to suggest a highly ferruginous 

 variety. In another and greyer specimen the matrix is largely 

 glassy, the more crystallized portions being gathered into little flecks 

 and patches visible to the naked eye. 



But the main interest rests with the examples, preserved thus 

 locally, of rocks which have been lost to us throughout this district 

 under the enormous outpourings of basalt. 



At the first glance many of the Ardtun pebbles recalled, in a redder 

 and altered form, the sanidine-lavas of Ischia or the Bhine. Their 

 microscopic examination fully bears out this view, which has been 

 confirmed by further evidence. One specimen, seen in section, has 

 the familiar pale augites, the abundant porphyritic felspars, the 

 fluidai glassy matrix that one associates with trachytic flows. The 

 specific gravity proves to be only 2*45, and the felspar, as determined 

 carefully by Szabo's method, contains more potash and less soda than 

 many accredited sanidines. 



Another more crystalline specimen consists of crowded felspars 



* Cf. Judd, Quart. Journ. Greol. Soc. vol. xxx. p. 228. 



t Prof. Jukes, in 1860, writes of a similar but far more ancient deposit: — 

 "It had very much the aspect ..... of one of the beds of volcanic breccia 

 and conglomerate one so often sees about recent and active volcanoes ; and it 

 occurred to me that in these pebbles of vesicular trap we might have preserved 

 the only fragments of the more superficial parts of the flows of molten matter.'" 

 ('• Igneous Eocks of Arklow Head," Journ. Greol. Soc. of Dublin, vol. viii. 

 p. 32.) 



