19 
" filmy wings. The rivers and shores were tenanted by 
" saurian s more or less amphibious, while the sea was full of 
" forms of zoophyta, mollusca, articulosa, and fishes ; un- 
" doubtedly the general impression gathered from all those 
" monuments of earlier creations is that they lived in a 
" warm climate." 
The limestone rocks of these periods are supposed by Dr. 
Forchhammer to owe their origin to ancient submarine 
springs of great force, yielding mingled carbonates of lime 
and magnesia, which were afterwards consolidated together, 
or separately deposited. Hence, also, again was the internal 
density of the earth being relieved and lessened. 
With the chalk ends the long series of deposits ranked as 
secondary strata. " From the mountain limestone to the chalk/' 
says Phillips, " it is impossible not to recognise, on a great 
" scale, the gradual change of the physical conditions of the 
" globe which took place during this period. Two very dif- 
" ferent assemblages of terrestrial plants had flourished and 
"become extinct. The ancient and abundant flora of the 
"carboniferous era, with its lepidodendra, sigillaria, and 
" calamites, had been replaced by new races of Zamiae and 
" Cycadese, which both in their turn vanished from the 
" northern zones of the earth before the completion of the 
"cretaceous system. The marine zoophyta were changed. 
"Two large assemblages of fishes had vanished, and the 
"gigantic reptile forms had come into being, reproduced 
"themselves to a marvellous extent, and then all perished 
" with the close of the v chalk, or the secondary period. "* 
* The inversion of the pole at the time of the carboniferous era 
would, as the sun left the northern hemisphere, leave the flora of this epoch 
to perish for want of heat. 
Another inversion would, at the termination of the Chalk period, by its 
severe winters (see p. 15) destroy not only the Zamiae, Cycadese, but the 
whole marine animal life of the northern hemisphere, as well as the numer- 
ous Reptilia of that period. 
b2 
