AITNIVEBSAEY ADDEESS OP THE PEESIDEH-T. 



77 



Hills, so well described by Mr. Teall, lias invaded the bedded 

 porphyrites of tbat region.^ A similar rock has been noticed by 

 ray brother. Prof. James Geikie, associated with the Lower Old Eed 

 Sandstone volcanic rocks of the east of Ayrshire. A remarkable 

 petrographical variety has been mapped by Mr. B. N. Peach, rising as 

 a small boss through the lower part of the great lava-sheets cf the 

 Ochil Hills above Tillicoultry. It is a granophyric quartz-diorite, 

 which, under the microscope, is seen to be composed of short, thick- 

 set prisms of plagioclase, with abundant granophyric quartz, a 

 pleochroic hypersthene, and needles of apatite. Sometimes the 

 pjToxene is replaced by green chloritic pseudomorphs." 



At the other end of the series come the felsites, quartz- porphyries, 

 minettes, vogesites, ' hornstones,' and ' claystones,' which have a 

 close-grained texture, often with porphyritic felspars, quartz, or 

 black mica, generally a whitish, pale buff, orange, pink, or purplish- 

 grey colour, and a specific gravity of about 2'55. 



Though I class these rocks as intrusive, I am not prepared to 

 assert that in none of the instances where they occur as sheets may 

 they possibly have been erupted at the surface as lavas. In one or 

 two cases the evidence either way is doubtful, but as the great 

 majority of the acid rocks can be shown to be intrusive in their 

 behaviour, T have preferred to keep them all in the same category. 

 I am prepared to find however that, as so vast an amount of 

 felsitic debris was ejected to form the tuffs, some of this material 

 may have flowed out in streams of lava. 



The following Table shows the chemical composition of some of 

 the acid sills and dykes : — 



Chemical Analyses of Acid Eruptive Rocks from the Lower Old 

 Eed Sandstone. By Prof. E. Fka^^klaxd. 





SiOg. 



73-91 

 64-73 



-04 

 70-28 



AI2O3. 



14-41 

 17-01 



1-01 

 12-54 



•76 

 2-35 



1-24 

 •43 



MnO. 



CaO. 



MgO. 



K2O. 



Na^O. 



P2O5. 



H2O. 



* Hornstone,' 

 Torgeith Knowe, 

 Peutlands 



•07 

 24 



... 



1-21 

 4-19 



•92 

 •91 



4-90 

 -66 



-52 



3-36 

 3-27 



3-92 



1-57 

 3-75 



5-84 



-26 

 •16 



•90 

 2-78 



1-99 



'Hornstone,' 

 Braid HiUs* 



Tinto, Lanark- 

 shire : 



Soluble in hydro- 

 chloric acid ... 



Insoluble in 

 ditto 









* This specimen also yielded 0-13 of ferrous oxide, and 2-42 of carbon dioxide. 



1 Geol. Mag. for 1883, pp. 100, 145, 252, and 'British Petrography,' 

 pp. 272, 278. ^ From notes by Dr. Hatch. 



