Yol. 6t > .'] the toadstones oe Derbyshire. 275 



ophitic olivine-dolerite, through a dolerite with large augite-grains 

 and small patches of ophitic structure, to a dolerite with granular 

 augite. 



The following brief description of the structure of the lava above 

 the dolerite will show the difference between the two types of rock 

 found in the sill and in the lava-flow. The lava a few inches from 

 the top and from its junction with the limestone (905) x is a much- 

 altered dolerite. with felspars of two generations and iron-oxide ; 

 there is no certain trace of augifce. Other portions (12, 11, 10) 

 consist of an amygdaloidal dolerite: the vesicles are filled with 

 crystalline calcite and chlorite; the olivine is altered to calcite 

 and chlorite ; the felspars are turbid and of two generations ; the 

 matrix consists of iron-oxide, or of very minute ill-defined felspar- 

 laths or microliths as well as iron-oxide. 



The intrusive rock contains veins or dykes of a finer-grained 

 rock. Some of these dyke-like masses are a partly-decomposed 

 ophitic dolerite (943, 944) ; others (903, 942) consist of a fragmental 

 rock, composed of small pieces of a more or less rounded green 

 chloritic material, red isotropic patches, and pieces of rock with 

 felspars in an iron-oxide base : the whole being cemented together 

 with dusty and reddish-brown fragments. In the former case, the 

 dyke-like structure may be simply due to joints in the ophitic 

 dolerite, and in the latter to the filling-in of joints by fragments of 

 the adjacent rock or of the lava above. 



Ible Sill (PI. XXI) is situated immediately to the east of the 

 village of Ible. It measures about half a mile from east to west, 

 and a third of a mile from north to south. Soon after passing Griff 

 Grange, a footpath leads up through a small ravine in the limestone 

 to the village of Ible. On emerging from the wood the path passes 

 over the junction, and enters the igneous rock. 



On the north-east, near Whitecliff Farm, the toadstone passes 

 under the limestone with a north-north-easterly dip of 20°, and near 

 the eastern boundary the limestone dips north-eastward. On the 

 south of the sill, on both sides of Griff-Grange Valley (a continu- 

 ation of the Via Gellia towards Winster), the limestones dip nearly 

 due west. About 200 feet of the igneous rock are seen on the same 

 horizon as the adjacent limestones : it evidently traverses the beds 

 of limestone, and is intrusive. Further proofs of the intrusion are 

 found in the partial or complete marmorization of the limestones 

 immediately to the south of the sill, and in its microscopic structure. 



The only type of rock that I have been able to find in the 

 intrusive mass is an ophitic olivine-dolerite, rich in olivine- 

 phenocrysts. The olivine occurs in large idiom orphic crystals 

 measuring up to 5-5 millimetres in length, and in groups or nests of 

 crystals. It is embedded among the felspars, and present in the 

 ophitic plates of augite. In the more altered portions of the rock 

 the olivine and augite are replaced by calcite. 



1 Numerals in parentheses refer to the numbers of the slides in the Author's 

 collection. 



v2 



