ON THE GOTHAM MARBLE 
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chalk ; the whole carefully dried under a weight of 3 lbs. 
and then cut into and examined. This presents a drying 
layer on a fluid one, but the black fluid does not rise into 
the cracks of the chalk cake at all ; i.e. there were neither 
hedge ” nor trees.” 
V. Mr. Thompson's Theory. — That the “ trees,” “ hedges ” 
and “ hummocky ” surface v/ere due to bubbling 
off of gases from the dark band in which the “ hedge ” 
arises, carrying up the dark stuff. 
Objections are the concentrically ringed “ branches,” 
often directed horizontally, or even downwards from the 
“ trees ” ; the presence of “ islands ” ; and especially 
the fact that in horizontal section the “ trees ” are seen to 
be not cylindrical columns but polygonal walls. More- 
over, his chief argument rests on a fallacy, for he lays great 
stress on the fact that the “ atmosphere ” rises to the 
“ trees.” So it does at their tops, but not near their bases, 
where it is usually horizontal, emd occasionally even falls a 
little. 
He advanced some experimental evidence, under some- 
what abnormal conditions, in support of his theory. I 
have modified these as follows : — take equal parts of 
oxalic acid in powder, soot, and sodium bicarbonate, mix, 
lay at the bottom of a basin, and smooth. Cover with a 
layer J-1 inch thick of precipitated chalk, and smooth. 
Pour on v/ater (over a filter paper, so as not to disturb). 
These conditions are I submit much more natural. Ebulli- 
tion takes place, and columns of black stuff are carried up 
and spread out on the top. Dij gently, v/hen sections 
can be cut. These show no resemblance to the Gotham 
Marble. The only hummocks formed are the outpoured 
soot. There is nothing like either “ hedge,” branches,” 
or polygonal walls. Thompson suggested that the hum- 
mocky surface was supported by air-bubbles, quoting the 
analogy of a copper-zinc couple under water, but surely no 
