314 LADY McEOBEET ON ACID AND OTHEE [Dec. 1914, 



lay claim to being phonolites themselves, as has been hitherto 

 thought, but are closely allied. Their content of alkali is high. 

 Soda-bearing minerals, such as riebeckite, segirine-augite, primary 

 albite, and soda-orthoclase, are well developed and play an important 

 part. Only a felspathoidis wanting to complete the analogy. 



I wish to take this opportunity of expressing my thanks to 

 Dr. Peach, Dr. Flett, and Mr. E. B. Bailey for much valuable 

 advice and assistance during the progress of the work, and also to 

 Mr. Gr. W. Tyrrell for the loan of rock-sections. 



EXPLANATION OF PLATES XLI-XLIII. 

 Plate XLI. 



[Except in one case, the slides belong- to the Author's collection.] 



Fig'. 1. Porphyritic sanidine -trachyte from the sheet below North Hill. 

 Ordinary light, X 32 diams. (Slide No. 21b.) The figure shows 

 porphyritic sanidine-crystals in a ground-mass of orthoclase-laths 

 and limonite. (See p. 307.) 



2. Fine-grained riebeckite-felsite from the southern slope of Wester Hill, 



100 feet below the summit. Ordinary light, X 37 diams. (Slide 

 No. 14.) The figure shows dark patches of riebeckite in a fine 

 felsitic ground-mass. (See p. 308.) 



3. Augite- olivine -trachyte from near the summit of Eildon Mid Hill. 



Ordinary light, X 18 diams. (Slide No. 14, Forbes Collection, 

 Hunterian Museum, Glasgow.) The figure shows subidiomorphic 

 crystals of asgirine- augite and small granular olivines in a mass of 

 alkali-felspar. (See p. 309.) From a slide kindly lent by Mr. G. W. 

 Tyrrell. 



4. Orthophyric riebeckite-trachyte from the northern slope of Mid Hill. 



Ordinary light, X 37 diams. (Slide No. 73.) The slide shows 

 stumpy crystals of alkali-felspar in interstitial plates of riebeckite. 

 (See p. 309.) 



5. Quartz-trachyte from Bemerside Quarry. Crossed nicols, X 37 diams. 



(Slide No. 49.) The figure shows orthoclase-laths arranged in 

 marked parallelism with interstitial patches of quartz. (See p. 306.) 



Plate XLII. 



Fig. 1. Sketch-map of the Eildon Hills, on the scale of 6 inches to the mile, or 

 1 : 10,560. The broken lines drawn between the main rock- types do 

 not represent definite geological boundaries, but have been inserted 

 merely in order to give a general idea of the distribution of the types 

 and the probable structure of the ground. 

 2. Section, on a horizontal and vertical scale of 6 inches to the mile, 

 drawn across the above map, along the line A A. 



Plate XLIII. 



General map, on the scale of 1*5 inches to the mile, or 1 : 42,240, showing- 

 the position of the various sills, dykes, and necks described in the 

 paper, and their relations to the Upper Old Bed Sandstone. 



