Presidential Address—The Geology of Devon. 009 
known in its vesicular form as “honeycomb dunstone,” was of a 
volcanic nature, had long ago been recognized by De la Beche; but 
it was the late Mr. John Arthur Phillips who first clearly demon- 
strated, in his classical papers on the Cornish “ greenstones,” that 
many of these beds were actually lava-flows. Mr. Rutley went a 
step further, and considered that he had found in Brent Tor a frag- 
ment of one of the old volcanic necks. His famous diagram, with 
its column of ashes flattened by the wind, described by himself as 
as “a chimera which may embody a certain amount of truth,” is 
familiar to all geologists. Nay so graphic was the picture, and so 
convincing the arguments, that a certain Mr. Thorpe fancied that 
he had corroborative evidence of the prevalence of the south-west 
wind in Devonian times, because, forsooth, he had found lapilli from 
Brent Tor in the joints of a limestone at Newton Abbot. 
Let us express a hope that before the Association next meets at 
Tavistock the boundaries between the Devonian and Carboniferous 
may have been made as clear as noonday, and accurately laid down 
on a six-inch map, which shall itself be a model of chartography. 
(2) We must now take into consideration the nature of the basic 
igneous rocks, commencing’ with those which are interbedded, most 
of which are now said to be of Devonian age. Before doing so it 
will be necessary to say a few words about the “killas,” a very 
loose term better understood by miners than geologists. Judging 
from Mr. Worth’s remarks on the stratigraphical relations of the 
Devonian rocks of South Devon, most of the “killas” of this 
district belongs to the grey and drab slates intersected by lodes and 
elvans, which was described by Conybeare as the metalliferous 
series : above this comes a group more variable in its nature, which 
is especially characterized by interbedded volcanic rocks, and Mr. 
Worth suggests that the Brent Tor series may belong to this group : 
ahove these again are the purple and green slates immediately under- 
lying the Plymouth limestone. The main point to notice is, that 
the whole of this slaty series is regarded as below the Plymouth 
limestone. Consequently it must belong to the lower part of the 
Middle Devonian, and possibly to even lower beds. Mr. Rutley, 
if I recollect rightly, regarded the Brent Tor series as possibly in 
the Upper Devonian. 
The interbedded basic igneous rocks, then, are placed by Mr. 
Worth a long way below the Plymouth limestone ; whereas the late 
Mr. Champernowne, in an interesting posthumous communication 
to the Geological Society, was disposed to regard his Ashprington 
voleanic series as above the main limestone of that district. In 
reference to this difference of opinion two points seem to present 
themselves for consideration. Firstly, that the schalsteins need not 
be confined to any particular horizon in so thick a series as the 
Middle Devonian ; secondly, that the phenomena of extravasation, 
whether interbedded or transgressive, is limited, with very un- 
important exceptions, to the southern portion of the county, from 
whence the line of igneous products may be traced into Cornwall. 
Hence the area of erupted rock is local, and to a certain extent 
