100 A NEW BRITISH ROCK. BY T. BARRON 



into the sandstone, lines of fusion are seen which much 

 resemble the fluxion-structure of a rhy elite. Round these 

 veins the quartz has been fused ; it is now in the form of 

 chalcedony. In the sandstone itself, mica is seen to be de- 

 veloping ; and there is a good deal of felspar, mostly unaltered 

 plagioclase showing lamellar twinning. 



Another specimen of sandstone, taken one foot below the 

 junction-line, had lost all traces of bedding, and broke with 

 an irregular fracture. Scales of a silvery mica were seen 

 lying in all directions and at all angles to the bedding- 

 planes, thus proving that they have been developed subse- 

 quent to the deposition of the sandstone. 



Under the microscope, the mica proved to be muscovite. 

 Fresh plagioclase has also been developed ; and the cementing 

 material between the sand-grains has been made to assume 

 a finely granular form. 



The presence of fresh plagioclase in a sandstone is not 

 an every-day occurrence, and it is necessary to find an ex- 

 planation to account for it. Sandstone, being a porous rock, 

 is not likely to contain the alkalies necessary for the forma- 

 tion of plagioclase felspar, because of their solubility in water. 

 They must, therefore, have been derived from another source, 

 that being the overlying igneous rock. It was noticed that 

 no ferro-magnesian mineral was observed in the overlying 

 rock ; but there was a good deal of iron oxide scattered 

 through it, which was shown to be derived from the alteration 

 of riebeckite in the Eildon rock. The mineral, being a soda- 

 amphibole, might, by its decomposition, supply the alkali 

 necessary for building up a felspar such as albite ; and I am 

 inclined to regard this as the true explanation of the presence 

 of fresh plagioclase in the sandstone. 



I have only to add that this work has been done in the 

 Geological Research Laboratory of the Royal College of 

 Science ; and to express my thanks to Professor Judd for 

 the assistance and many valuable suggestions that he has 

 given me. 



