ANNIVERSARY ADDRESS OF THE PRESIDENT. 



75 



s of PorpTiyrites from the Lowe^- Old Red Sandstone of 

 Scotland. By James S. Grant Wilson, Esq. 





SiOg. 



AI,03. 



Fe^Oa. 



FeO. 



MnO. 



CaO. 



MgO. 



KoO. 



NasO. 



H2O. 



H0SO4. 



Losa.i 



Sea wd Law ... 59-29 



16-30 



I 



77 



3-70 



-41 



4-81 



3-15 



4-19 



3-44 



3-84 





Rennieston ... 62-81 



16-40 





65 



3-27 



•81 



4-46 



1-64 



3-60 



3-02 



4-04 







Cunrieston ... 63-38 



15-77 





73 



2-65 



-08 



4-44 



1-88 



1-88 



4-54 



4-69 







Duncan'sDubs' 59-44 



1615 



1 



05 



2-83 



•37 



6-70 



2-46 



3-18 



3-70 



3-35 







Whitton Hill 60-70 



17-98 





63 



2-58 



-20 



7-07 



2-20 



3-57 



2-95 



3-45 







Cuddies' Tops 



60-58 



12-25 



1 



01 



4-13 



•15 



4-40 



2-86 



2-19 



3-61 





•55 



2-15 



Cocklawfoot. . 



62-29 



17-03 





93 



2-44 



•21 



3-92 



2-71 



1-14 



3-20 



-29* 



•37 



4-81 ■ 



Morebattle ... 



59-82 



16-96 





20 



6-57 



•15 



4-73 



2-84 



2-63 



3-04 





trace 



1-98 



* This is CO2. 



The microscopic structure of the porphyrites of the Lower Old 

 Red Sandstone has been partially investigated, especially those of the 

 Cheviot Hills, by Mr. Teall ^ and by Dr. Petersen.^ Much, however, 

 still remains to be done before our knowledge of this branch of 

 British petrography can be regarded as adequate. The groundmass 

 in some of the rocks consists mainly of a brown glass with a streaky 

 structure (as in the well-known variety of Kirk Yetholm) ; more 

 usually it has been devitrified more or less completely by the 

 appearance of felspathic microlites until it presents a confused fel- 

 spar aggregate. The porphyritic felspars are often large, generally 

 striped, but sometimes including crystals that show no striping. 

 They are frequently found to be full of inclusions of the base, and 

 these sometimes consist of glass. The ferro-magnesian constituents 

 are usually rather decomposed, being now represented by chloritic 

 pseudomorphs, but augite, and perhaps still more frequently ensta- 

 tite, may be recognized or its presence may be inferred among them. 

 Magnetite is commonly traceable, and apatite may be occasionally 

 detected. As the result of decomposition, ealcite, ohlorite, and 

 limonite are very generally diffused through the rocks.^ 



(c) The lavas which may be separated as Trachytes offer no 



^ Geol. Mag. for 1883, pp. 100, 145, 252. 



^ ' Mikroskopische und chemische Untersuchungen am Enstatit-porphyrit aus 

 den Cheviot Hills,' Inaug. Dissert. Kiel, 1884. 



Descriptions have also been published of detached rocks from other districts, 

 such as those by Prof. Judd and Mr. Durham of specimens from the Eastern 

 Ochils, Quart. Journ. Geol. Soc. vol. xlii. (1886) p. 418. 



3 I have again to thank my colleague. Dr. F. H. Hatch, for the notes on the 

 microscopic structure of the rocks with which he has furnished me for the 

 second part of this Address. 



