NATURE NOTES 
of the whole modern science of geology — we must, in fact, study 
several very different ways in which rocks are in process of 
formation to-day. Broadly speaking, however, these fall into 
two classes : the unstratified, crystalline, non-fossiliferous class 
of rocks has resulted from a state of fusion by slow cooling, 
in a fashion more or less similar to what we can see in the 
lava-flows of any volcano ; whilst the stratified, granular, fossil- 
iferous rocks are the result of the mere drying of a series of 
muds or precipitates. 
Stratification. 
It is difficult to realise when we visit beach after beach, and 
see shingle thrown up in ridges there, a wide stretch of sand here, 
and clayey mud not far off — that a little further out to sea there 
is a much greater uniformity in the character of the sediment. 
Conversely we are apt at first to think of onr stratified rocks, 
such as our Oolitic limestones or our Greensands, as extending 
farther without variation in mineral constitution than they do. 
In interpreting even a small section of ordinary sedimentary rock, 
such as some alternating thick beds of limestone “ freestone ” 
and thin beds of “ flagstone,” with some sand below them, and 
a thick series of clay-beds below that, it is difficult to realise the 
element of time, to remember that slow changes in the relative 
level of the sea-floor and the land may practically at successive 
periods of time have made tiuit particular part of the earth’s 
