78 PEOCEEDINGS OE THE GEOLOGICAL SOCIETr. 



The rock of Tinto, which may be considered typical of the pre- 

 vailing felsitic, acid intrusive rocks of the series, presents several 

 slightly different varieties. Dr. Hatch informs me that as the result 

 of his examination of a number of microscopic slides prepared from 

 specimens taken by me from various parts of the hill, some are minettes 

 showing small isolated crystals of orthoclase and rare flakes of biotite, 

 sometimes granules of quartz, imbedded in a brown, finely microlitic 

 groundmass of felspar powdered over with calcite ; while other 

 specimens have a granular instead of a microlitic groundmass, and 

 contain a considerable amount of quartz in addition to the consti- 

 tuents just mentioned. A conspicuous knob on the south side of 

 Tinto, called the Pap Craig, is a mass of augite-diorite, which has 

 risen through the other rocks.^ The sills in the same region show 

 still further differences. Some are true ' felspar-porphyries,' and 

 ' quartz-porphyries' varying in the relative abundance and size of 

 their porphyritic orthoclase and quartz, while others, by the intro- 

 duction of hornblende or pseudomorphs after that mineral, pass into 

 vogesites. 



3. Tu-ff^s Sf Aciglomerates. — The fragmental materials ejected from 

 or filling up the vents range from the finest compacted dust up to 

 some of the coarsest agglomerates in this country. In general 

 they consist mainly of detritus of porphyrite, and have been derived 

 from the blowing up of already consolidated masses of that rock. 

 The fragments are generally angular, and range from minute grains 

 up to blocks as large as a cottage. The tuffs are often more or less 

 mixed with ordinary non- volcanic sediment, and as they are traced 

 away from the centres of eruption they pass insensibly into sand- 

 stones and conglomerates. 



But while, as might be expected, the tuffs are most commonly made 

 up of debris of the same kind of lavas as those that usually form the 

 sheets which were poured out at the surface, they include also bands 

 of material derived from the destruction of much more acid rocks. 

 Throughout the chain of the Ochil Hills, for example, in the midst 

 of the bedded porphyrite-lavas, many of the thin courses of fine tuff 

 consist largely of felsitic (rhyolitic) fragments, with scattered felspar 

 crystals. The most remarkable examples of this nature, however, 

 are to be met with at the great vent of the Braid Hills and in the 

 chain of the Pentland Hills, which runs south-westward from it. 



^ This rock differs considerably from the other intrusive masses in its neigh- 

 bourhood. Dr. Hatch has found it to be composed chiefly of lath-shaped 

 striped felspar, with some granular augite, magnetite, and interstitial quartz. 



