SiRATIGRAPHlCAL FEAIURES. ^3 



That the eruptions were accompanied by explosive outbursts is proved 

 by the intercalation of tuffs and bands of breccia, produced by the 

 shattering of previously consolidated flows, among the bedded lavas. 



The viscid character of such acid lavas as these, when they were in 

 a liquid state, is often well exhibited by the uneven and scoriaceous 

 aspect of the surface of the flows, and by the abrupt manner in which 

 they frequently die out. Some of the sheets are of considerable thick- 

 ness and extend without any alteration in appearance for several miles, 

 but very often a scarp can be seen to be built up of a number of 

 thin sheets, differing from each other in colour and texture. A colum- 

 nar structure is frequently well developed in them, and also serves to 

 distinguish the different sheets, building up a scarp, from each other. 



There are indications that during this period the eruptions were 

 not altogether of an acid character. In several places sheets of igneous 

 rock of a more basic type, containing plagioclase felspar and no free 

 quartz, are interbedded with the rhyolites. It is difficult to say 

 whether these are truly interbedded lavas or whether they are sills thrust 

 in between the sheets of rhyolite after the consolidation of the latter. 

 The material of which they are composed is much more subject to 

 decomposition than that of the rhyolites, and as a rule their relations 

 with the latter are obscured by soil and debris. They sometimes 

 appear to be intrusive, but in other cases the appearance of inter- 

 stratification is quite as clear. On the whole I think that the balance 

 of evidence is in favour of their being intrusive. 



The tuffs and breccias associated with the lavas are quite subordi- 

 nate in amount to the latter, so far as can be seen from the limited 

 exposures now visible. It is quite possible that a great part of the 

 sandy plains between the hills of lava may be underlaid by these softer 

 rocks, which are as a rule exposed only in the slopes at the base of 

 the scarps formed of the harder lavas. The tuffs are usually dis- 

 tinctly stratified, but the breccias are generally found in small patches 

 only, enclosed between sheets of lava. The bands of pebbly conglo- 



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