Vol. 50.] STRUCTURE OF CARBONIFEROUS OOLERITES AND TUFFS. 611 



Micldleton, Outcrop 61. — On the road to Rider Point I have found 

 no exposure, the ground being covered with grass. 



North of Fairfield Common, near the old racecourse (outcrop 

 13), a small strip of toadstone is mapped by the Geological Survey. 

 Every exposure shows limestone. A small quarry has lately been 

 made for the mending of walls. The section exposed shows 1 foot 

 of soil, 2 feet of clayey soil containing a few lumps of toadstone, and 

 then limestone in almost horizontal beds. 



I am aware that the above evidence, except that regarding out- 

 crops 11 and 35, is of a negative kind, and that the fact of my being 

 unable to find the outcrops does not prove that they never existed. 

 It is possible that some of them may have been exposed at one 

 time, and that the exposures are now hidden. 



I have added to the list three outcrops not mapped by the 

 Geological Survey. 



Brook Bottom, Outcrop 7, is mapped by them as one bed. It 

 is clearly two — a tuff and a lava separated by beds of limestone. 

 See under Tuffs, no. 7 a. 



Litton, near Tideswell, Outcrop 8. — A lava and a tuff separated 

 by limestone. See under Tuffs, no. 8 6. 



Potluck, Outcrop 60. — This is exposed in a field in front of the 

 rifle-target, and is probably continuous with that at Pittlemere, 

 no. 6. Both are ophitic olivine-dolerites. 



In addition I have examined specimens from Wakebridge Mine and 

 from the following mine-heaps : — Glory Mine, Crich ; Elton, "Wheel 

 Pake, and Black Hillock : and have descended three mines, viz.., 

 Mill Close, Wheel Pake, and Wakebridge. Only in the last have I 

 seen the toadstone in place. In the New Key Cavern at Matlock 

 Bath I have seen blocks of toadstone which were not in situ. I 

 have also examined several specimens of the rock from other places 

 which were not in situ. My object has been to examine every occur- 

 rence of the rock in detail from as fresh specimens as possible. 



Por convenience, this paper is divided into two parts : The Lavas 

 (none of the rock having yet been proved to be intrusive) and The 

 Pragmental Pocks or Tuffs. 



Part I. — The Lavas. 

 1. Olivine. 



Olivine occurs mostly in the form of phenocrysts and groups or 

 nests of crystals. The phenocrysts vary in size from 5 millim. in 

 length, down to the smallest measured, *06 x "08 mm. Of fifteen large 

 ones measured, ten are over 2 mm. in length, four over 3 mm., and 

 one over 1 mm. The outline is generally very well marked, giving 

 the usual six-sided sections with acute angles between the domes. 

 Sections parallel to (100) and (010) are found showing a positive 

 and negative bisectrix, and others perpendicular or nearly per- 

 pendicular to an optic axis. The outline is often well preserved 

 when the olivine is altered to oxide of iron or some other mineral, 



Q. J. G. S. No. 200. 2 u 



