Yol. 6$.^\ THE TOADSTONES OF DEBBYSHIRE. 277 



the cracks (thin slices 187, 315, 316, 317, 675) ; in others (671, 681, 

 100, 314) it is altered to the Potluck pseudomorph. The felspars 

 are generally small. 



Some specimens show the transition from the ophitic to the 

 suhophitie type. Slice 670 contains some large ophitic plates of 

 augite, and small augites with a few felspar-laths sticking into them, 

 with small patches of interstitial matter containing magnetite-rods 

 and minute felspar-microliths. The granular type (307) consists of 

 olivine altered to the Potluck pseudomorph, with small granules 

 of augite and felspar-laths. The olivine-crystals are small, reach- 

 ing a length of only 1 mm., and the biggest augite-granule measures 

 about 16 mm. In some cases (312) the felspars are bigger, and 

 reach a length of 1*25 mm. Another specimen (376) combines 

 the subophitic and granular structures. It is mainly a dolerite 

 with granular augite, but contains one small ophitic plate of that 

 mineral. 



The rock at the upper margin near the Methodist Chapel is much 

 decomposed (1137). Pseudomorphs of olivine and altered felspar- 

 laths are present. The olivine often has been replaced by a quartz- 

 like mosaic, and the rock is similar in many respects to the upper 

 margin of the Peak-Forest sill. 



There are two other small outcrops of toadstone, which complete 

 the list of intrusive rocks of the county. 



High-Peak Tavern (PI. XIX). — On the road between Peak 

 Forest and Tides well is a small outcrop of igneous rock which is 

 probably intrusive. It is a very fine-grained dolerite or basalt. 

 The groundmass consists of small felspar-laths, and small grains and 

 prisms of augite. The felspars sometimes attain a length of 0*5 

 millimetre, and the augite-grains and prisms a length of 0-05 mm. 

 The olivine is in idiomorphic crystals, and is unaltered except along 

 the cracks : one crystal measures 1-25 mm. in length. The rock is 

 in a very fresh condition, and (so far as can be ascertained) cuts 

 across the strike of the limestones. 



Low Farm (PL XXI). — Immediately south-west of Low Farm 

 near Bonsall is a small exposure of olivine-basalt, which cuts 

 across the strike of the limestone-beds to the south-east of it. It 

 does not appear to be connected, either with the Bonsall sill, or with 

 the lower lava of the Matlock area. The olivine-phenocrysts attain 

 a length of 0*5 millimetre, and are often unaltered, except along 

 the cracks. The felspar-laths are of about the same length as the 

 olivine-crystals. The groundmass consists of magnetite in small 

 cubes and crowds of augite -prisms, the largest of which measures 

 about 0*028 millimetre in length, and a felspathic or colourless 

 glassy material without microliths. 



The following table embodies a comparison of the structures of 

 the intrusive masses described in the preceding pages. The smaller 



