Vol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES AND TUFFS. 633 



contour-lines. On another small inlier of limestone, between the 

 900 and 1000-feet contour-lines near Low Farm, I found in the 

 walls pieces of a similar ashy limestone. I have been unable at 

 present to look carefully for any outcrop. 



At Jughole Wood the dip is northerly ; between there and Low 

 Farm it is north-easterly. It is likely that the anticlinal seen in the 

 Derweut Valley, opposite the High Tor, passes near Low Farm. If 

 this be so, there may be two outcrops of limestone- tuff forming parts 

 of the same bed under the dolerite, and we should not expect to find 

 any signs of this bed of tuff in other parts of the outcrop of toadstone, 

 because they consist only of the upper part of the bed. 



A specimen of the ashy limestone (sp. gr. 2-57) contains also 

 pebbles or pieces of limestone. Examined under the microscope, it 

 consists of lapilli in a limestone containing organisms. A large 

 lapillus contains olivine (altered to a green-and-yellow dichroic 

 material) and felspars with nearly parallel extinction, in a dusty 

 brown base. Others contain vesicles filled with a substance 

 having fibro-radial structure, lighter in colour than the remaining 

 parts of the lapilli, which are mostly isotropic. Sometimes the 

 outer portions of the lapilli are clear yellow and isotropic, the inner 

 portions being crowded with black enclosures and giving bright 

 colours in polarized light; in some cases they are fibrous, the 

 fibres being in bundles. Many amygdaloids occur, showing the 

 black cross as the nicols are rotated. Some vesicles are filled with 

 calcite, some with a black material, and others with the cement of 

 limestone. The lapilli are irregular in shape, and are often very 

 delicate in form. They appear to have fallen into a limestone in 

 process of formation, or into a limestone-paste. 



Another specimen (sp. gr. 2-61) contains a large lapillus about 

 1 inch in diameter. Olivine-pseudomorphs and felspar-laths occur in 

 a black base. There is a large quantity of crystalline calcite which 

 may be altered portions of the base, or which has more probably filled 

 in the numerous and large vesicles. Often half a dozen patches 

 which do not communicate have the same optical orientation, 

 extinguishing together, and contain parallel sets of cleavage-cracks. 

 In some parts the calcite is more plentiful than the black base, but 

 the latter is always a connected whole, the walls of the cells being 

 very narrow, except in a few cases where we have a smaller lapillus 

 in a vesicle of the large piece. Other lapilli are of a dirty brownish- 

 yellow colour, isotropic in parts, and in others having a roughly 

 spherulitic kind of structure. The cement is a reddish-coloured 

 substance which contains small pieces of limestone with organisms 

 in them, and also probably small fragments of organisms. Here 

 the lapilli are mixed with fragments of already formed limestone, 

 and do not appear to be embedded in a limestone-paste as those in 

 the previously described specimen. 



Grange Mill, Outcrop 46. — This is well exposed near the junction 

 of the roads to Winster and Ible, beyond the top of the Via Gellia. 

 The exposure is for some 15 yards on the roadside, and may be 

 traced in the fields on the right for about 100 yards. 



