Yol. 55.] PAULTED IXLIER IN TIDESWELL DALE. 247 



approached : some feet below its junction with the overlying lavas 

 it. is similar to the fine-grained dolerite at the bottom of the sill, 

 and cannot be distinguished from it in a hand-specimen. 



This part of the sill in which the (juarry is worked is about 

 60 feet thick. It is covered by 20 to 30 feet of vesicular lava, 

 and rests on the 2 feet of vesicular rock seexi in the year 1870 

 by Mr. Mello, but now covered by debris. The microscopical 

 details given above not only show that the rock is coarsely crys- 

 talline in the centre, and becomes fine-grained near its upper and 

 lower surfaces, but also that the sill may be divided into five bands 

 or zones. The central portion near the top of the quarry consists of 

 a band of coarsely crystalline, ophitic dolerite at least 6 feet thick, 

 and having a specific gravity of 2-80 to 2-83 ; above and below it are 

 bands of the type with granular augites and large felspars, having 

 a specific gravity of 2-80 to 2-84. The lower of these bands attains 

 a thickness of about 11 feet. Below this is a band of fine-grained 

 dolerite about 14 feet thick, and having when fresh a specific 

 gravity of 2-85 to 2-88. The uppermost band is a fine-grained 

 dolerite, similar to that which constitutes the lowermost band. 



The rock in the quarry is traversed by numerous small veins of 

 a mineral which is probably chrysotile. It is of a golden yellow, 

 and consists of prisms or bundles of parallel fibres arranged per- 

 pendicularly to the walls of the cracks in which it occurs. While 

 wet it is soft and easily rubbed into a waxy material between the 

 fingers, but when dried becomes tougher and slightly brittle. It 

 has a rough cleavage parallel to the longitudinal axes of the fibres. 

 Under the microscope it is dichroic, has straight extinction, and the 

 minor axis of depolarization coincides with the direction of the fibres. 



(5) The Vesicular Lava above and below the Sill. 



The vesicular rock (sp. gr. 2-24) above and below the intrusive 

 dolerite differs from it in microscropic structure and specific gravity. 

 Fone of the minerals are in so fresh a state of preservation. The 

 olivines are smaller, and are replaced by various decomposition- 

 products. The felspars are turbid and much altered. Sometimes 

 there are traces of augite in small, more or less altered grains. 



A specimen from the upper portion on the eastern slope of the 

 valley, above the cottages (sp. gr. 2-72), has the appearance of a 

 fragmental rock. One part of the slice is made up of separate 

 fragments of a highly vesicular rock, containing felspar-laths and 

 microlites in a base which has little reaction on polarized light. A 

 secondary felspar-like mineral, which is biaxial and sometimes 

 twinned, fills the cracks and larger vesicles. This specimen may be 

 from the broken upper surface of a lava-flow. (PI. XX, fig. 4.) 



(c) The Columnar Clay, 



Several thin slices of the columnar clay were examined, and, as 

 might be expected, they exhibit no very definite structure under the 

 microscope. They are of an uneven reddish-brown colour, with 



