Vol. 63.'] THE TOADSTONES OE DERBTSHIRE. 279 



portion of agglomerate, and the Hopton Vent is a breccia of basalt- 

 fragments. The contemporaneous lavas are vesicular and amygdal- 

 oidal in structure, and often very much decomposed. They contain 

 olivine, augite, and felspars, magnetite and oxide of iron. The 

 felspars are often present in two generations, namely, in big 

 elongated crystals with broken ends and in small laths. The olivine 

 is generally altered to serpentine and oxide of iron, the augite is 

 seldom fresh. The intrusive rocks or sills are for the most part 

 ophitic olivine-dolerites, and pass from a very coarse-grained dolerite 

 through the intervening stages to a fine-grained dolerite or basalt. 

 These dolerites are similar in structure to those of Tertiary age 

 described by Prof. J. W. Judd. 1 



EXPLANATION OF PLATES XIX-XXII. 



[These four plates are geological maps of the areas in which the toadstones 

 appear at the surface, on the scale of 2 inches to 1 mile (the Ashover inlier 

 excepted). The topography is based on the 6-inch Ordnance-Survey maps, and 

 the boundaries of the Millstone Grit, Yoredale rocks, and Mountain Limestone 

 are taken from the 1-inch Geological-Survey maps, except that in Pis. XXI 

 & XXII slight deviations in the Mountain-Limestone boundaries have been 

 made. The dips are inserted from my own observations.] 



Plate XIX. 

 The Miller' s-Dale or north-western area of volcanic activity. 



Plates XX & XXI. 

 The Matlock or south-eastern area of volcanic activity. 



Plate XXII. 

 The Tissington or south-western area of volcanic activity. 



Discussion. 



The President recalled the attention of the Meeting to some of 

 the interesting incidents connected with the history of the inves- 

 tigation of the Derbyshire toadstones. It was Whitehurst who, in 

 his ' Inquiry into the Original State & Formation of the Earth,' 

 published in 1778, first recognized the volcanic origin of these 

 rocks, which he declared to be ' as much a lava as that which flows 

 from Vesuvius or Etna.' Faujas de St. Fond visited the ground in 

 1784, after an interview with Whitehurst in London, who showed 

 him his specimens of the toadstones and appealed to the French 

 traveller's extensive experience of volcanic countries. Faujas had 

 evidently made up his mind, from an examination of these speci- 

 mens, that the toadstones were only varieties of ' trap,' which, like 

 the Wernerians, he looked upon as an aqueous deposit. Hence, 

 when he came to Buxton with this prepossession, though he noted 

 the remarkable resemblance of some of the toadstones to true lavas, 



1 Quart. Journ. Geol. Soc. vol. xlii (1886) pp. 49-95. 



