A NEW BRITISH ROCK. BY T. BARRON 95 



certain places it is much contorted. At one place it deviated 

 sharply from the horizontal line, and was traced in an almost 

 vertical direction for 20 or 30 feet, where it was lost on account 

 of the steepness of the rock. 



On the south side of the hill, the rock is of a dull, reddish- 

 pink colour, and is much more decomposed than that on the 

 north side. The felspars have, in manj' cases, been dissolved 

 out, leaving their casts, which are sometimes partly filled 

 with chalcedony. This character is, however, not very constant; 

 for in certain places the rock is hard and compact, ringing under 

 the hammer and breaking into thin slabs. 



Petroqraphical Description. 



The rocks from Middle Eildon are all of felsitic texture. 

 On a fresh fracture a few cleavage-faces of a clear felspar may 

 be seen ; and by the aid of a lens a bluish-green mineral can be 

 recognized all through the base of the rock. In some of the 

 specimens a banded structure is seen ; this is accentuated by the 

 segregation of iron oxide along the bands. 



Under the microscope, the banded rock shows a trachytic 

 structure ; it is of a non-porphyritic character, only a few large 

 sanidines being scattered through it. These show irregular 

 outlines ; are often corroded ; and when examined between 

 crossed nicols, break up into a granular aggregate. This 

 arrangement of the larger felspar crystals is characteristic of 

 this lava-flow. The sanidine crystals also show the irregular 

 series of cracks at right angles to their direction of elongation, 

 which is so characteristic of them in all lavas. The base of the 

 rock is made up of felspar microlites, a bluish-green mineral, 

 and a colourless mineral of low refractive index and double 

 refraction. 



The bluish-green mineral occurs in small pieces, often grouped 

 together, and moulding the felspars in the same way as augite 

 does in many of the basic rocks. Between crossed nicols these 

 patches extinguish together, showing that they are parts of the 

 same crystal. Many of the pieces show cleavage-lines parallel 

 to their sides, and one or two likewise show the hornblende 

 cleavages. The extinction angle, measured from the longi- 

 tudinal cleavage-lines, was 5". When rotated over the 

 polarizer this mineral showed intense pleochroism. Owing to 

 the small size of the pieces I was unable to obtain a satisfactory 



