640 ME. H. H. ARNOLD-BEMROSE ON THE MICROSCOPICAL [Nov. 1 894, 



right the layers are well seen. Those forming the roof become 

 detached, and fall to the floor by their own weight. They are about 



1 inch thick. In the roof is a somewhat rounded block, measuring 

 7x5x4 inches, of decomposed dolerite. The olivine is replaced 

 by iron oxide, and the felspars are much altered. Under the micro- 

 scope no augite is seen, though it cannot be said that it was never 

 present. I also found a nearly spherical block, 6 inches in diameter, 

 of a similar dolerite containing amygdaloids. Many limestone- 

 pebbles are found in this tuff. 



In the cutting near Ashover the thickness of the exposed beds is 

 about 16 feet. Near Fall Mill a shaft was sunk 70 yards through 

 this rock. 1 The rock dips east under the limestone ; for about 



2 yards below the junction it is powdery, and passes into a more 

 or less laminated rock traversed by numerous veins of calcite and 

 containing fragments of chert, limestone, and dolerite. It is purple 

 and green in colour. The rock is so decomposed that it is difficult 

 to obtain a good piece for a thin section. 



A specimen (sp. gr. 2*49) consists of dirty-looking green lapilli 

 in a cement of calcite and grey material. Under the microscope 

 the lapilli are light and dark brown or very pale green, all isotropic. 

 Some contain pseudomorphs of olivine, often in groups ; few contain 

 felspar-laths with parallel extinction. The vesicles are generally 

 filled with calcite. 



Another specimen (sp. gr. 2*46) consists of green lapilli with 

 dark green amygdaloids in a red cement. Under the microscope 

 the lapilli are very light green. They contain no crystals, with the 

 exception of two which have a few felspars. In plane-polarized 

 light the groundmass gives a lively play of colours, and sometimes 

 black brushes opening out into rude hyperbolas, not unlike those 

 seen in a biaxial crystal in convergent light. 



Some are elongated and have elongated vesicles, which are filled 

 with a material similar to that of which the lapilli are composed, 

 and show a black cross due to radial arrangement of fibres. The 

 lapilli are often bordered with a black substance, and some are 

 entirely black throughout. The cement consists of smaller lapilli, 

 often isotropic, with a little calcite. 



A third specimen (sp. gr. 2-35) contains lapilli which vary greatly 

 in size. The majority are very small and fantastic in shape, contain 

 no crystals, and are generally isotropic. Some are altered to calcite, 

 except on the outside border. (See PI. XXV. fig. 6.) 



The fantastic outlines of the lapilli show that they cannot have 

 been formed by the trituration of a compact lava. They are differ- 

 entiated from the substance of the solid rock in the dolerites and 

 basalts of the district by their preponderating glassy base more or 

 less altered, the presence in it of a large number of skeleton-crystals 

 and crystallites, and by their numerous amygdaloids. Their form 

 and structure prove that tbe5" are true volcanic ejectamenta, and not 

 the product of broken-up lava-streams. They vary in magnitude 

 from very small fragments up to about the size of a pea. 

 1 Geol. Survey Mem., N. Derbyshire, 2nd ed. 1887, p. 154. 



