ANNIVERSAKY ADDRESS OF THE PRESIDENT. 79 



These rocks are generally pale flesh-coloured or buff in tint, and 

 compact in texture. The finer varieties are so compact as to 

 present to the naked eye no recognizable grains ; they might be 

 mistaken for felsites, and indeed, except where they contain recog- 

 nizable fragments of rock or broken crystals of felspar, they can 

 hardly be discriminated. They consist of exceedingly fine com- 

 pacted felsitic dust- Here and there, however, the scattered crystals 

 of felspar and small angular fragments of felsite which may be 

 detected in them increase in number until they form the whole of the 

 rock, which Is then an angular tuff or fine volcanic breccia, made up of 

 different felsites or rhyolites, among which, even with the naked eye, 

 delicate flow-structure may be detected. In these pale acid tuffs 

 fragments of different porphyrites may often be observed, which 

 increase in number as the rocks are traced away from the main vent 

 of the Braid Hills. 



At my request my colleague, Mr. George Barrow, has determined 

 the silica percentages in a few specimens which I have selected as 

 showing some of the more characteristic varieties of these tuffs. 

 His results are exhibited in the following Table : — 



Percentage of Silica in some Tuffs from the Braid and 



Pentland Hills. 



Silica 

 percentage. 



1. Quarry above Woodhouselee 63*3 



2. South-west side of Castlelaw Hill 73-15 



3. Quarry on road, | mile N.E. of Swanston 



(Braid Hill vent) 74-1 



4. South-west side of Castlelaw Hill 75*0 



5. Castlelaw Hill 76-00 



6. South side of White Hill Plantation 90*00 



From these analyses it may be inferred that the average amount 

 of silica in the more typical varieties is between 70 and 75 per cent. 

 The last specimen in the Table, with its abnormally high percentage 

 of acid, must be regarded as an exceptional variety, where there has 

 either been an excessive removal of some of the bases, or where 

 silica has been added by infiltration. 



The microscopic examination of these rocks has not added much 

 to the information derivable from a study of them in the field. In 

 their most close-grained varieties they are hardly to be distinguished 

 from felsites. But they generally show traces of the minute par- 



