ON THE GOTHAM MARBLE 
137 
than the atmosphere, and stand out in weathered speci- 
mens. The “ atmosphere ” is light coloured and usually 
of coarser texture ; it is horizontally banded, the bands 
usually rising to the “ trees ” at their tops, a point on 
which Thompson lays great stress. But at the bottoms 
of the trees they do not so rise, and occasionally even 
slope down a little. 
(d) “ Islands.'^ In the upper part of the “ atmosphere,” 
dark circles, J inch across, are often found which are not 
attached to any “ ^ree,” but are completely isolated from 
every other dark band. This can be proved by dissecting 
them out, and examining the cube so extracted on each 
of its six sides and observing no dark bands in it at all. 
The importance of these will appear later. 
(e) The “ CanopyT This presents a hummocky upper 
surface, the arrangement being not so much in round 
bosses as in interlacing ridges. The layers composing 
it resemble the basement beds. The lowest ones are 
thick between the trees, and thin over them. Unless 
the canopy is fractured, the trees rise under the hummocks, 
not between them. 
The Horizontal Section (fig 1) Shows the surprising fact 
that the “ trees ” are not cylindrical columns, but the 
expression, in section, of vertical walls arranged poly- 
gonally. I have examined hundreds of specimens for 
this point, and find it quite constant. Indeed, many of 
my specimens show it better than the one figured. 
Texture. — The most characteristic feature of the Gotham 
Marble is its peculiar texture. The colour, almost con-^ 
choidal fracture, hardness, and very fine grain and high 
polish are unmistakable. It is an important point that 
the “ atmosphere ” always shows these features least per- 
fectly, and often not at all, being sometimes an ordinary 
-shelly limestone. 
Thompson says the “ trees ” are usually aragonite, 
