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



chiefly of rounded or subangular fragments of the lava of the district 

 in which the tuff lies, and many of them do not differ in any respect 

 from the substance of the solid rock as seen in sheets or dykes at 

 the surface. The Derbyshire lapilli are seldom rounded or subangular 

 fragments, but answer rather to those of palagonite from Kilmundy 

 Hill and other localities, and like them have no counterpart amongst 

 the lavas erupted at the surface. 



EXPLANATION OF PLATES XXIV. & XXV. 



[The figures are all photographed from the microscope in ordinary light. 

 The first four are magnified 40 diameters, and the last eight 50 diameters.] 



Plate XXIV. 



Fig. 1. Peak Forest pseudomorph after olivine, Outcrop 4. Near the top 

 of the figure is a small nucleus of fresh olivine, surrounded by a feebly 

 double-refracting substance, of which the nebulous patches are also 

 composed. The pseudomorph along the cracks is yellow and slightly 

 dicbroic ; the portions showing cleavage are yellow for rays vibrating 

 parallel to the short axis of the polarizer, and green for rays at right 

 angles to that axis. Both portions polarize in bright colours, and 

 extinguish parallel with the length of the original olivine. . 



Fig. 2. Potluck pseudomorph after olivine, Outcrop 60, with cleavage parallel 

 to its length, and showing the position of the cracks in the original 

 olivine. It is enclosed in ophitic augite. It is yellow for rays 

 perpendicular to the short axis of the polarizer, and orange-brown 

 for those parallel to that axis. 



Fig. 3. Potluck pseudomorph after olivine, Outcrop 60. The greater portion 

 is yellow, with traces of cleavage parallel to its length. It is dichroie, 

 and polarizes in colours of the first and second orders. Two smaller 

 portions on the left are cut parallel to the cleavage-planes and show 

 an acute bisectrix (negative) with a very small axial angle. To the 

 right are plagioclase-laths in ophitic augite. 



Fig. 4. Potluck pseudomorph after olivine, Outcrop 30. The double refraction 

 is strong, and the polarization-colours are similar to those of biotite. 

 It is green for rays vibrating parallel to the short axis of the polarizer, 

 and pale yellow for those at right angles to that axis. The cracks 

 are filled with iron oxide. Above and below it are two similar, but 

 smaller pseudomorph s. 



Fig. 5. Portion of a large lapillus from Brook Bottom tuff, Outcrop 7. In the 

 left-hand portion the felspars are broken across. They are embedded 

 in a dense black groundmass, which contains several amygdaloids of 

 calcite. 



Fig. 6. Lapillus from Grange Mill tuff, Outcrop 46. The internal portion has 

 a slight action on polarized light. The border is of a coffee-brown 

 colour and contains globulites. Smaller lapilli make up the greater 

 portion of the rock immediately surrounding it. 



Plate XXV. 



Fig. 1. Tuff, Monk's Dale, Outcrop 16. On the right is part of a lapillus with 

 an isotropic base. On the left is a fragment of limestone containing 

 organisms. 



Fig. 2. Tuff, Monk's Dale, Outcrop 16. Amygdaloid lapillus with an isotropic 

 base. Some of the crystallites have no action on polarized light, 

 others have a slight action and extinguish parallel to their length. 



Fig. 3. Lapillus from Castleton tuff, Outcrop 1, containing several felspars. 

 It is of a black colour and cracked. The cracks are filled with, and 

 the lapillus is bordered by a light yellow material, probably an 

 alteration-product. The cement is crystalline calcite. 



