76 



PiROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



distinctive features externally by which they may be distinguished 

 from the porphyrites. Indeed, both groups of rocks appear to be con- 

 nected by intermediate varieties. In the Cheviot Hills some of the 

 lavas are found, on microscopic examination, to contain a large 

 admixture of unstriped porphyritic felspars, which can sometimes 

 be recognized as sanidine in Carlsbad twins. The groundmass is 

 sometimes a brown glass, but is usually more or less completely 

 devitrified, portions of it being enclosed in the large felspars. 

 Chlorite pseudomorphic after augite or cnstatite may be detected, 

 and sometimes a brown mica. A specimen of one of these rocks, from 

 a locality to the north-west of Whitton, near Jedburgh, was found 

 by Mr. J. S. Grant Wilson to have the following composition : — 



Chemical Analysis of a Trachyte from the Lower Old Red 

 Sandstone. 





SiOa. 



A],0,. 



Fe^Os. 



FeO. 



1-8 



MnO. 

 •25 



CaO. 



1-84 



MgO. 

 1-37 



K2O. 



5-02 



NagO. 



HoO. 



Total. 



N.W. of Whitton 

 Hill, Jedburgh, 



(No. 1938) 



8p. gr. 2-55. 



62-44 



18-99 



3-35 



2-65 



2-48 



100-19 



(d) Acid rocks such as Pelsites and Ehyolites are extremely rare 

 among the lavas poured out at the surface during the time of the 

 Lower Old lied Sandstone. The only nodular felsite of this age 

 which I have met with is that of Lough Guitane among the ' Dingle 

 Beds,' near Killarney, to which reference will be made in a later 

 part of this Address. As already stated, some acid lavas may 

 possibly be included in the great felsitic tuff-bands of the Pentlands 

 and among the sills of the same chain. 



2. Intrusive Sills, Sfc. — While the interbedded lava-sheets are 

 mainly porphyrites, the intrusive rocks are generally much more 

 acid, and may be grouped under the convenient head of felsites. 

 Some intrusive porphyrites and even more basic rocks do indeed 

 occur in dykes and sills as well as also filling vents. But the 

 rule remains of general application over the whole country that 

 the materials which have consolidated in the volcanic orifices, or 

 have been thrust among the rocks in dykes, bosses, or sills, are 

 decidedly acid. In this series of rocks a greater range of types 

 may be traced. At the one end we find true granites or granitites, 

 as in the intrusive bosses of Spango AVater and of Galloway, 

 which, for reasons which I will afterwards adduce, may with 

 some probability be assigned to the volcanic history of the Lower 

 Old Eed Sandstone period. The augite-granitite of the Cheviot 



