Yol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES AND TUFFS. 631 



Miller's Dale Station, Outcrop 19. — This bed is mapped as run- 

 ning from Cressbrook through Taddington and Chelmorton to Great 

 Low. The rock is generally an olivine-dolerite, and in places an 

 olivine-basalt. East of Miller's Dale Station a cutting has lately 

 been made for a tram-line, which runs from the down line of rail- 

 way past the bottom of several large limekilns to the large lime- 

 stone-quarry above the toadstone. 



The junction of the two rocks is well exposed. Besting on the 

 limestone are about 1 or 2 feet of a clay and a decomposed rock ; 

 above this the rock becomes hard and breaks readily into laminae. 

 It then becomes harder and less platy, and is altogether about 

 2 feet thick. It is very fine-grained and of a drab colour. Ex- 

 amined under the microscope, it is seen to be a crystalline limestone 

 with few, if any, organisms, and containing small lapilli. These 

 are often clear and glassy (isotropic), with globulites and sometimes 

 a felspar having parallel extinction. Sometimes they are brown, and 

 contain hair-like crystallites, the whole being isotropic. Some are 

 bordered with iron oxide. A few fragments of felspar showing 

 a biaxial figure, and pieces of calcite twinned like a felspar, 

 probably pseudomorphs, occur in the limestone. These are in a 

 specimen 6 inches above the clay (sp. gr. 2 - 46). 



A specimen 9 inches above the clay (sp. gr. 2*52) is similar, 

 except that the lapilli are smaller, form a smaller proportion of the 

 whole mass of the rock, and are often altered to calcite. 



A block of amygdaloidal rock (sp. gr. 2 - 62) embedded in this 

 ashy limestone contains pseudomorphs of olivine, large and small 

 felspar-laths, skeleton-crystals and rhombic sections of felspar in a 

 dark base, which is partly isotropic. Above the ashy limestone we 

 find a large, irregular-shaped mass of hard, grey, amygdaloidal rock 

 several feet in thickness (sp. gr. 2-58), similar to the block previously 

 described, many of the felspars in it having parallel extinction ; 

 there is also a felt of crystallites with magnetite-skeletons in the 

 partly glassy base. About 9 feet above the clay this rock appears 

 more like an ordinary dolerite, and under the microscope the felspars 

 are seen to be larger, more numerous, and much less altered, and 

 there is a less amount of base. 



Above this dolerite is a bedded coarse-grained tuff, the lapilli being 

 dark, with amygdaloids of calcite. Under the microscope the lapilli 

 may be divided, into two kinds : (a) pseudomorphs of calcite after 

 olivine, felspar-laths mostly altered to calcite, and patches of calcite 

 in a dark groundmass coloured with iron oxide ; (6) yellow lapilli, 

 having little action on polarized light, and sometimes bordered with 

 iron oxide, some containing no crystals, and others felspar-laths. 

 The cement often consists of the same yellow substance with mag- 

 netite-dust. The limekilns, and the roads to them, are between the 

 coarse tuff and the spheroidal dolerite above it, so that the junction 

 is not seen. 



Several hundred yards east of the kilns is another quarry in which 

 the ashy limestone and the dolerite above it may be seen ; both 

 are much more decomposed than in the exposure previously described. 



