Vol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES AND TUFFS. 629 



much, more altered. The lapilli vary much in size, they are yellow 

 and brown in colour, often isotropic, but sometimes have a feeble 

 action on polarized light. Some of them are entirely altered to 

 calcite, others have a border of the isotropic material or of iron 

 oxide, whilst the interior is composed of calcite. The vesicles are 

 tilled with calcite or with a clear felspar-like mineral, or again with 

 a brown substance which gives a more or less regular cross in 

 parallel polarized light. There are pseudomorphs of olivine in a 

 few lapilli. 



Four of the included blocks have been examined microscopically. 

 Their specific gravity ranges from 2-72 to 2-49. They are very 

 similar one to the other, so that a general description will suffice. 

 The olivine is altered either to a brown, partly transparent and 

 partly opaque substance, or to a clear felspar-like mosaic, the 

 portions of which are not large enough to test by convergent light. 

 The felspar occurs in small lath-shaped sections, which often have 

 parallel extinction, and are often arranged with their long axes 

 parallel to the sides of an olivine-section. A porphyritic crystal of 

 felspar occurs which is very much corroded. The extinctions are 0° 

 and 15° referred to the trace of the plane of composition ; like many 

 of the olivine-pseudomorphs, the crystal is surrounded by a darker 

 portion of the base, with few or no felspars. The base is sometimes 

 dark, and contains flakes, rods, or skeleton-crystals of magnetite. 

 In other cases the base is a brown glass, more or less cloudy. 

 The amygdaloids often have a ropy or knotted vermicular structure. 

 The ropy part is calcite, or the clear felspar-like material, or both 

 together ; the interior is calcite, or the felspar-like mosaic. Some- 

 times iron oxide fills a vesicle. 



Dove Holes, Outcrop 12. — This is mapped as toadstone by the 

 Geological Survey, and well described in the Memoir as " a crumbly 

 bed, pale grey with green specks, and contains pebbles of limestone, 

 one of which was seen as big as a man's fist." l It is doubtful 

 whether the rock is an igneous product. It is very much decomposed 

 and lies between two beds of limestone : I have, however, been able 

 to find a piece hard enough for a thin section. Under the microscope 

 there are no signs of altered felspars or of altered lapilli ; the rock 

 has, in fact, the appearance of a clay. Were it not for the presence 

 of rounded lumps of limestone similar to those at Ashover and in 

 other tuffs, I should class it as a clay. If a tuff, it is so much altered 

 that no sign of the original structure remains. 



Monk's Dale, Outcrop 16. — This rock is situated where the road 

 from Tideswell to Hargatewall crosses Monk's Dale. It is difficult to 

 find, the outcrop being small, and for the most part covered with 

 grass. In a field, near the footpath to Wormhill, may be found in 

 the soil rounded pieces of limestone and of a rock like limestone 

 containing a few small lapilli. An exposure, about a foot square, 

 on the road' to Hargatewall, consists of coarse and fine tuff, 



1 Geol. Surv. Mem., N. Derbyshire, 2nd ed. 1887, p. 21. 



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