144 
ON THE GOTHAM MARBLE 
combined with a little evaporation, precipitating the 
carbonate held in solution by that gas. It is not due to 
evaporation alone beyond the because it nearly com- 
pletely lines the pool. 
Experiment with a solution of carbonate of lime held by 
carbon dioxide, and warmed to 35° for an hour, shows how 
a little carbonate can thus be precipitated, especially if 
evaporation is allowed. 
Now the Gotham Marble may have been deposited (1) in 
an estuary, as in the Rhone deposit, or (2) in an evaporating 
Dead Sea, or (3) in a very shallow lagoon by a process like 
that at Nice and Tintagel. I incline to the last hypothesis. 
A study of the other members of the Rhaetic series has 
convinced me that they are not estuarine, and the abundant 
life, and rarity of gypsum and salt, preclude a Dead Sea. 
This subject will be returned to later. 
II. Explanation of the^ “ Hedge- Structured — This is 
formed as follows (referring, for simplicity’s sake, to the 
calculus). After a period of free deposition, the calculus is 
smooth, hard and spherical. Then comes a period of 
scanty deposition. The bands laid down are therefore 
darker (see above). These thin impure bands, when first laid 
down, are not hard and solid like the foundation they rest 
on, but have to contract to attain their final condition of 
firmness. But as the calculus will not yield, they have to 
crack into squares. When the next coat is laid down, it 
has to follow the surface it finds, and rounds off the angles 
and exaggerates the hummocks. Every fresh layer follows 
suit, provided it is very thin. , 
It may be objected that the individual “ hummocks ” 
are too far apart for such an origin in the case of Gotham 
Marble. But they are sometimes even farther apart in 
calculi. We have no data to show us how far away from 
one another the hummocks would contract in setting. 
Probably, too, some of them are dissolved off entirely in 
