CHAP. XXIX 
THE DERBYSHIRE TOADSTONES 
I I 
tlie ground, I was enabled to visit in rapid succession those tracts which 
seemed most likely to furnish the required evidence, and in a few days was 
fortunate enough to obtain proofs of six or seven distinct vents, ranging 
from the extreme northern to the furthest southern boundary of the volcanic 
district. Mr. Bemrose has undertaken to continue the investigation, and will, 
I trust, work out the detailed stratigraphy of the Carboniferous Limestone so 
as eventually to furnish an exhaustive narrative of the whole volcanic history 
of Derbyshire. Meanwhile no adequate account of the area can be given. 
But I will here state all the essential facts which up to the present time 
have been ascertained. 
1. the hocks erupted. — Mr. Allport has described the microscopic 
character of some of the toadstones , 1 and further details have been 
supplied by Mr. Teall . 2 The fullest account of the subject, however, is 
that given by Mr. Bemrose in the paper above referred to. This observer 
distinguishes the lava -form from the fragmental rocks, and gives the 
minute characters of each series. He does not, however, separate true 
interstratified lavas from injected sills, nor the bedded tuffs from the coarse 
agglomerates which fill up the vents. These distinctions are obviously 
required in order that the true nature and sequence of the materials in the 
volcanic eruptions may be traced, and that the phenomena exhibited in 
Derbyshire may be brought into comparison with those found in other 
Carboniferous districts. But to establish them satisfactorily the whole 
region must be carefully re-examined and even to some extent re-mapped. 
The lavas (including, in the meantime, sheets which there can be little 
doubt are sills) show three main types of minute structure and composition, 
which are discriminated by Mr. Bemrose as — (a) Olivine-dolerites ; these, 
the most abundant of the series, consist of augite in grains, olivine in 
idiomorphic crystals, plagioclase giving lath -shaped and tabular sections, 
and magnetite or ilmenite in rods and grains ; (l) Opbitic olivine-dolerites, 
consisting of augite in ophitic plates forming the groundmass, in which are 
imbedded idiomorphic olivine, plagioclase (often giving large lath-shaped 
sections and magnetite or ilmenite) ; (c) Olivine-basalts ; these rocks are dis- 
tinguished by containing crystals of augite and olivine in a groundmass of 
small felspar-laths, granular augite and magnetite or ilmenite, with very 
little interstitial matter. They have been noticed only in two of the out- 
crops of toadstone. 
The fragmental rocks have been shown by Mr. Bemrose to cover a 
much more extensive space than had been previously supposed. He has 
found them to be distinguished by an abundance of lapilli varying from 
minute fragments up to pieces about the size of a pea, and composed of a 
material that differs in structure from the dolerites and basalts with which 
the tuffs are associated. These lapilli consist largely of a glassy base more 
or less altered, which is generally finely vesicular and encloses abundant 
skeleton crystals and crystallites. The tuffs thus very closely resemble 
1 Quart. Journ. Geol. Soc. xxx. (1874), p. 529. 
2 British Petrography, p. 209. 
