British Association^ Edinhurgh Meeting — Lapiooi'th. 229 
ways, many "formations." A formation, which is the unit of geological 
stratigraphy, is a rock sheet composed of many strata possessing com- 
mon lithological characters. The formation may be simple, like the 
Chalk, or compound, like the New Red sandst ne; but, simple or com- 
pound, local or regional, it must be always recognizable, geographically 
and geologically, as a lithological individual. 
As regards the natural grouping of these lithological individuals as 
such, fair progress has been made of late years, and our information is 
growing apace. We know that there are at any rate three main groups: 
1st, the stratifled formations due to the action of moving water above 
the earth-crust; 2d, the igneous formations which are derived from be- 
low the earth-crust; 3d, the metamorphic formations which have un- 
dergone change within the earth-crust itself. We know also that of 
these three the only group which has hitherto proved itself available for 
the purpose of reading the past history of the globe is that of the strati- 
fied formations. 
Studying these stratified formations therefore in greater detail, we 
find that they fall naturally in their turn into two sets — viz., a mechanical 
set of pebble beds, sandstones and clays formed of rock fragments 
washed ofE the land into the waters, and an organic set of limestones, 
chalk, &c., formed of the shells and exuvite of marine organisms. 
But when we attempt a further division of these two sets, our classifi- 
cation soon begins to lose its definiteness. We infer that some forma- 
tions, such as the Old Red and the Triassic, were the comparatively 
rapid deposits of lakes and inland seas; that others, like the Coal Meas- 
ures, London clay, &c., were the less rapid deposits of lagoons, river 
valleys, deltas, and the like; that others, like our finely laminated shales 
and clays of the Silurian and Jurassic, were the slower deposits of the 
broader seas; and finally, that others, like our Chalk and Greensand, 
were possibly the extremely slow deposits of the more oceanic deeps. 
Nevertheless, after looking at the formations collectively, there re- 
mains no doubt whatever io the mind of the geologist that their me- 
chanical members are the results of the aqueous degradation of vanished 
lands, and that their organic members are the accumulated relics of the 
stony secretions of what once were living beings. Neither is there any 
possibility of escape from the conclusion that they have all been depos- 
ited by water in the superficial hollows of the sea-bottoms and ocean 
floors of the earth-crust of their time. 
In the life of ever}^ individual stratified formation of the mechanical type 
we can always distinguish three stages: first, the stage of erosion and 
transportation, in which the rock fragments were worn off the rocks of 
the higher ground and washed down by rain and rivers to the sea; sec- 
ond, a stage of deposition and consolidation below the surface of the 
quiet waters; and third, a final stage in which the completed rock for- 
mation was bent and upheaved, in part at least, into solid land. In the 
formations of the organic type three corresponding stages are equallj'' 
discernible: first, the period of mineral secretion by organized beings; 
second, the period of deposition and consolidation; and third, the final 
