389 
ve have no knowledge of such growth, capable of producing 
h on° i maSSeS that make up the vast beds in question. ° 
~U. I here is another very well-known formation which 
may also serve to remind us of the past influence of that 
mysterious power of life upon our globe. We acknowledge 
at coal is the result of vegetation, with but little realization 
ot the stupendous luxuriance of growth that must have been 
required to produce the thick seams of coal that we have so 
extravagantly dealt with, yet before we think we have grasped 
the problem of the world, we should be prepared to show 
whence all that wealth of carbon was derived, whether it was 
all previously existent in the atmosphere as carbonic acid, and 
what must have been the effect of its withdrawal. I may here 
remark that all calculations made with a view of proving the 
length of time that has passed in the formation of the various 
s rata, from the rate at which similar formations take place 
at the present moment, are vitiated by the impossibility of 
proving that the conditions were absolutely identical with those 
w nci we are observing. We know that a comparatively small 
difference in the depth of water is sufficient to put a stop to 
ie growth of coral, and that the variations of temperature that 
ie zoophyte can bear are very limited, but we do not yet 
know how rapidly it is possible for the coral formation to *go 
on when all the conditions are favourable, and specially when 
the supply of the requisite carbonate of lime is abundant. The 
same remarks are true of the chalk formation : it may be that 
what we are now able to observe of ocean life is but a faint 
survival of the teeming vitality that has been supported in the 
sea in past ages, the records of which are written in the vast 
chalk-beds. The clay deposits of which we have spoken give 
us another example of this uncertainty. 
21 It is very tempting to say an inch of deposit has been 
lormed in a year, therefore if the deposit is 1,000 inches thick 
it is 1,000 years old ; yet nothing can be more fallacious. We 
see the stream in summer running perfectly clear from the 
spring on the moor, bringing down no deposit at all, but, on the 
contrary cutting its way through the mud previously brought 
down. . A thunderstorm passes over the moor, and in a few 
hours it is pouring down a muddy stream, carryiug more sand 
and clay in a minute than a century of its former current could 
lave moved; and if it change thus from hour to hour, how can 
we foim even a slight idea of what effect the tremendous 
changes of climate, of which we see traces, have had on the 
