Vol. 67.] ASSOCIATED ROCKS OF NORTH-EAST MONTGOMERYSHIRE. 447 



made up of parallel layers of roughly hexagonal columns, each column 

 sloping a few degrees from the horizontal. The walls separating 

 the layers are almost vertical, dipping about 80° to the east of south- 

 east (see PI. XXXV, fig. 2). The rock is fine-grained and light 

 green in colour, with a specific gravity of 2-72, when fairly fresh. 

 Under the microscope the structure is trachytic : the rock is seen to 

 consist of small idiomorphic plagioclase-felspars arranged in lines 

 of flow, with a certain amount of chloritic ground-mass, and a few 

 tiny grains of augite where the rock is fairly fresh. Small patches 

 of chlorite (probably ripidolite) show a radiating arrangement, when 

 examined under the higher powers of the microscope (see fig. 8, 

 below). Some grains of magnetite, pyrite, and graphitic material 

 occur ; while calcite becomes so abundant near the walls of the 

 dyke, that the rock will effervesce with acid. The last-named 

 mineral has evidently been derived from the calcareous breccia or 

 conglomerate, which is seen resting against the eastern face of the 

 dyke. The chlorite is probably the decomposition-product of a 

 ferromagnesian mineral. There is an entire absence of porphyritic 

 constituents, which were also absent primarily. 



Fig. 8. — Bostonite of the Welshpool DyJce, showing radiating 

 aggregates of chlorite- crystals. 



[Magnified 50 diameters.] 



The rock is somewhat similar to that of the intrusive dykes of 

 Cefn and the Breiddens, where the original ferromagnesian mineral 

 was described by Prof. Watts as being hypersthene [4] [7] [12]. 

 These rocks have also been discussed by Mr. H. S. Jevons [11], who, 



