Yol. 6$.'] THE TOADSTONES OF DERBYSHIRE, 271 



presence bv parallel faults or intrusion. According to Sir Archibald 

 Geikie, 1 to whom I showed the band on the western side of the 

 valley, there is no trace of a fault, and the bands are either ' inter- 

 calated lava-strearns or intrusive sills.' The microscopic structure 

 of the dolerite is like that of a lava-flow, and unlike that of the 

 Derbyshire sills. A careful examination of the ground shows that 

 the stream passes through an anticline in the shales and limestones, 

 the beds of limestone between the dolerites being often vertical. 

 The limestones and shales dip on either side of the valley west- 

 ward and eastward, at angles up to 60°. It is, therefore, likely that 

 the two bands form part of the same bed, brought up by a pericline 

 through which the stream has cut its way. The limestones above 

 and below the lava show no signs of marmorization. 



The lava to the west rests upon that line-grained dark variety of 

 crystalline limestone which is so common in the Limestone-Shales. 

 It is followed by a coarser limestone consisting of fossil-fragments. 



A short distance west of Shaw's Farm, north of Tissington, 

 a vesicular and amygdaloidal rock is seen in two small knobs 

 or hillocks, between which a small hollow runs down to the Farm. 

 On the east the southern part of the outcrop disappears under the 

 limestone, which in turn is succeeded by shales. The limestone 

 probably belongs to the Yoredale Series. I have been unable to 

 trace the rock for any noticeable distance, and consider that it 

 forms part of a lava-flow. 



Y. The Intrusive Dolerites or Sills. 



The intrusive rocks or sills are confined to the Mountain Lime- 

 stone (maps, Pis. XIX & XXI), and with three exceptions consist 

 of ophitic olivine-dolerite. Near the margins they generally pass 

 into a dolerite with granular augite, or a rock composed of felspar- 

 laths and olivines in a more or less cloudy base. 



Peak-Forest Sill (PI. XIX). — This intrusive mass is an 

 ophitic olivine-dolerite which passes into a fine-grained dolerite at 

 its upper margin, and is best seen near Mill Cottage in Dam Dale. 

 Contact-metamorphism in the limestone above the sill may be seen 

 for some distance on the east and west sides of the valley. 



Sir Archibald Geikie, 2 in referring to this sill, says that 



' the limestone above a coarsely crystalline dolerite has been converted into 

 a white saccbaroid marble for about 2 yards from the junction.' 



• The sill may be traced westward as far as the road from Peak 

 Forest to Peak-Forest Station, and on the north it extends in a 

 narrow strip through the village and round to the north of Snelslow 

 Plantation, where it is carried by an easterly dip below the limestone. 

 A small patch of the same rock occurs a short distance north of 



* 'Ancient Volcanoes of Great Britain' vol. ii (1897) p. 17. 

 a Ibid. p. 22. 



