Prof. T. G. Bonney—On the Ightham Stone. 297 
lation of 12000 feet of Coal-measures in South Wales would involve 
a very material increase of pressure on the underlayers, | am of 
opinion that numerical calculations in these matters are only too apt 
to mislead by throwing a glamour of apparent mathematical accuracy 
over problems concerning which the most noteworthy feature is our 
profound ignorance. As in so many questions connected with the 
physics of the earth’s crust our data here are too scanty and too 
indefinite to make numerical calculations of much value. Who shall 
presume to assign quantitative shares (-+ or —) to (1) contraction 
due to metamorphism in the solid underlayers; (2) expansion of 
the rocks under increment of temperature; (38) the formation of 
geo-clines by lateral pressure; (4) contraction and expansion on 
melting and solidification ; (5) the effects of pressure on the water gas 
contained in the fluid magma; (6) the differential load on a flexible 
erust ? I for one will not. It is presumption enough in me to 
have ventured at all among the cross-currents of so difficult a sea, 
where to steer a mathematical course is impossible, and the best one 
can hope for is to keep one’s craft afloat. 
III].—Nore on THE StRucTURE OF THE IGHTHAM STONE. 
By Pror. T. G. Bonney, D.Se., LL.D., F.R.S., F.G.S. 
OME time since a student of University College, Mr. J. Hale, of 
Ivy Hatch, brought to me specimens of an extremely hard 
green sandstone, which he informed me came from the Folkestone 
Sand stage near Ightham, in Kent. As the microscopic structure 
proved rather interesting, I lately visited the locality in his company, 
and had the additional advantage of being conducted by Mr. B. 
Harrison, of Ightham, so well known for his discoveries of palzo- 
lithic implements and for his minute knowledge of the geology of 
the neighbourhood. 
The rock is briefly noticed in Mr. Topley’s excellent Survey 
Memoir on the Geology of the Weald, in the following terms (p. 
140) :—“ At Ightham, near the Roman Camp, is a hard white sand- 
stone five feet thick, a good deal like the ‘ Greywether’ sandstone of 
the Tertiary, and there is too another kind of stone, not observed 
elsewhere, a hard and tough dark green sandstone or rather grit. 
This was not seen in place, but on Ightham Common it is found in 
large masses, and is there called Ightham Stone and sometimes Fire- 
stone, from its being sufficiently hard to strike fire well.” 
The white sandstone is well exposed at the top of the northern 
escarpment of the commanding elevation of Oldbury Hill, and its 
cragey outcrop forms a part of the defence of the ancient camp. 
The rock appeared to me to vary in thickness and to be somewhat 
lenticular in its mode of occurrence.! Beneath the camp it is under- 
lain by soft ferruginous sand, but a short distance to the south and 
a few feet below, another mass of sandstone crops out, which how- 
ever is not nearly so hard. This seems to increase rapidly in thick- 
ness, and to be soon full ten feet thick, and perhaps more. On the 
1 Mr. Harrison informs me it is about 140 feet above the top of the Kentish Rag. 
