518 Alexander Scott — The Crawford/} ohn Essexite. 



Dark aggregates of ferromagnesian microlites suggest the original 

 presence of porphyritic mica or hornblende. The groundmass is 

 generally felsitic and, from the refractive index, seems to contain 

 a fair amount of orthoclase, a fact borne out by the quantity of 

 potash in the analysis. This rock has been analysed, the results 

 being given in column 1, table iv. In chemical composition it 

 closely resembles the albite-diabase of Trusham, Devon (column 2), 

 while it also shows some similarity with the keratophyres. The 

 former rock consists mainly of broad rectangular crystals of alkali- 

 felspar with interstitial chloritic material, and is therefore very like 

 the crystalline aggregates in the Craighead rock. In the neighbour- 

 hood of Abington there are several outcrops of lavas and intrusive 

 rocks of Ordovician age, and it seems probable that the rock in 

 question is related to these. The felspars are so fresh that they must 



TABLE IV. 



Si0 2 



Ti0 2 



A1 2 3 



Fe 2 3 



FeO 



MnO 



MgO 



CaO 



Na 2 



K 2 



H 2 + 



H 2 0- 



P 2 5 



C0 2 



100-42 100-31 100-27 100-61 



1. Altered lava(?), Craighead, analyst A. Scott. 



2. Albite-diabase (felspathic type), Trusham, Devon, analyst E. G. Eadley 



(includes -07 CI., -08 BaO, -03 FeSa). 1 



3. Keratophyre, Hamilton Hill, Peebles, analyst J. J. H. Teall. 2 



4. Keratophyre, Blankenburg, Harz (includes -11 SOs). 3 



have been completely recrystallized and their composition and form 

 suggest some infiltration of material from the essexites. The 

 felspars of the diabases of Devon and Cornwall are usually albitized, 

 a phenomenon which also occurs in the Southern Uplands. The 

 relatively basic nature of this mineral in the Craighead rock may be 

 due, as suggested above, to the introduction of material during the 

 metamorphism. This renders the determination of the original nature 

 of the rock difficult, but it seems probable, from a consideration of 

 the chemical analyses, that it was originally a felspathic diabase or 

 proterabase, resembling a trachyte in composition and having 

 porphyritic crystals of felspar and either mica or hornblende in 

 a fine-grained or glassy matrix. 



A thin band in the greywacke above the quarry appears to have 



1 J. S. Flett in Geology of Neivton Abbot (Mem. Geol. Surv.), 1913, p. 62. 



2 J. J. H. Teall in The Silurian Rocks of Scotland (Mem. Geol. Surv.), 

 1899, pp. 88-9. 



3 Quoted in Rosenbusch, Elemente der Gesteinslehre, 3rd ed., 1910, p. 343. 



