112 



PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



(d) Trachytes. — Some of the most remarkable lavas to be found 

 in any of the Plateaux are those which constitute a large part of 

 the Garlton Hills. They overlie the lower porpliyrite and basalt 

 platform which surrounds them as a narrow belt, while they occupy 

 the central and much the largest part of the area. They have been 

 included among the porphyrites, but are pale rocks, generally with a 

 yellowish crust, presenting when quite fresh a grey, compact, felsitic 

 base with large porphyritic crystals of felspar. 



A number of specimens selected by me as illustrative of the 

 different varieties were analysed by Dr. E. Erankland and Mr. Grant 

 Wilson, and their results are stated in the subjoined table. The 

 specific gravity of the rocks is about 2*6. 



Analysis of 



Bedded Trachytes from 



the Garlton Hills} 







SiOa- 



A\p,. 



Fe^Og. 



FeO. 



MnO. 



0-205 



CaO. 



2-58 



MgO. 

 0-74 



K2O. 



4-02 



N-aoO. 

 6-49 



0-801 



Total. 



100-186 



Pepper Craig ... 



62-61 



18-17 



0-32 



4-25 



Kae Heughs ... 



61-35 



16-88 



0-41 



5-01 



0-26 



2-39 



0-44 



6-12 



5-26 



1-70 



99-82 



Phantassie 



62-66 



13-37 



7-88 



0-98 



2-11 



0-85 



11-12 



Loss. 

 i-d6 



100-63 





59-85 



25-41 



7-25 



3-46 



0-45 



1-43 1 2-24 

 6-47 





100-09 



HopetounMon™', 



61-84 



20-72 



8-i^5 



2-15 



0-32 





100-45 



The microscopic characters of these rocks have not yet been worked 

 out, but I have asked Dr. Hatch to undertake the task, and we may 

 expect soon to have a full description of them from him. His pre- 

 liminary examination shows them to be well-marked and wonder- 

 fully fresh sanidine-trachytes, containing large crystals of perfectly 

 unaltered sanidine, sometimes also oligoclase. In some of the sheets 

 small but well-formed crystals of yellowish-green augite, in addition 

 to the porphyritic felspars, are imbedded in a fine groundmass 

 composed chiefly of microlites of sanidine, but with granules 

 of augite and magnetite plentifully interspersed, and occasionally 

 prisms of apatite. In others there is little or no ferro-magnesian 

 constituent. Other trachytes rather less basic than the augite- 

 bearing varieties here referred to, but resembling the second group, 

 occur as bosses in the Garlton Hills district, and are referred to in the 

 following section (e). 



The first two analyses are by Mr. Wilson, the last three by Dr. Frankland. 



