114 J. H. Teall — Cheviot Quartz- Fehites and Augite- Granites. 



down to the S.E. of this exposure, so that in all probability it forms 

 an extensive tract in this district. Examination with a hand lens 

 enables one to recognize easily a pink potash felspar, a light-coloured 

 almost white plagioclase, biotite and quartz. The felspars sometimes 

 measure ^ of an inch across, and so give the rock a slight porphyritic 

 aspect. They may be easily isolated and examined by Szabo's 

 method. I give below the results of my own examination of these 

 felspars and also those obtained by myself and Dr. Szabo on some 

 typical felspars. 



Examination of Felspars by Szabo's Method. 





1st Experiment. 



2ud Experiment. 



3rd 

 Experiment. 



Na,0 



K,0 



Fusibility. 



NasO 



KoO 



Fusibility. 



NajO 



K2O 





1-5 



1—3 



1—4 



1—5 



1—3 



1-5 



1—5 



1—4 



A(T) 



4—5 







3—4 



4—5 







4—5 



5 



1—2 



B(S) 



4-5 







3—4 



4-5 







4—5 



4—5 



1-2 



C(S) 



3-4 







2—4 



3—4 







3—5 



3—4 



1—2 



D(T) 



3 







2—3 



4 







4 



4—5 



1 



E(T) 



4 







2—3 



4 5 







4 



5 



1 



F(T) 



3 



2 



2—3 



3 



2-3 



4 



4 



3 



G(S} 



3—4 



2 



2 



3 4 



2 



4 



3 4 



3 



A. Oligoclase Twedstrand (Teall). 

 Ji. Oligoclase (Szabo). 



C. Andesine (Szabo). 



D. Andesine, Scomie Dyke (Teall). 



JE. Plagioclase, granite, Staindrop Rigg (Teall). 



F. Orthoclase, granite, Staindrop E.igg (Teall). 



G, Perthite group (Szabo). 



Mr. Waller, of Birmingham, appears to have been the first 

 observer who applied Szabo's method to the practical determination 

 of minerals in this country, and I am indebted to that gentleman 

 for instruction in the working of the method. There is no doubt 

 that it is a quick and reliable method for obtaining very valuable 

 information as to the nature of the constituents of rocks. The above 

 experiments prove clearly that the plagioclase belongs to the oligo- 

 clase-andesine group and the potash felspar to the perthite group of 

 Szabo ; a group which is characterized by the presence of from 4 

 to 6 per cent, of soda.' 



The two felspars occur in about equal proportions in the rock 

 from Staindrop Rigg. Augite and magnetite may be recognized 

 under the microscope, but they are far less abundant than in the 

 syenitic-looking varieties from Linhope Spout. Quartz on the other 

 hand is more abundant. The microstructure of the rock is thoroughly 

 granitic. Narrow contemporaneous veins of quartz and felspar with 



1 In is interesting to note that the augite -syenites and their porphyi-itic equivalents 

 of the Chvistiania district contain felspar rich in soda which appears to be sometimes 

 monoclinic and sometimes triclinic, with a cleavage angle diii'ering but slightly from 

 90°. Brogger has described these two forms in great detail (Die Silurischen 

 Etagen 2 uud 3, p. 203, et seq.). He names them soda-orthoclase and soda- 

 microcline. 



