the Distribution of Life and of Rocks. 409 
riods; and therefore it is necessary to fix the ages of the faults 
to interpret accurately the sequence of rocks, and to discover 
therefrom the old physical geography. 
13. From these considerations it follows that no deposit can 
be traced over a large area. And when the mineral character 
changes in a succeeding deposit, it follows that, at one end or 
the other, there will be no change of mineral character. Hence 
deposits cannot be identified or correlated over wide areas by 
this means. But this limitation of kinds of rock-material is 
evidence of change in physical conditions; and if uniformity of 
physical conditions can be determined, then it follows that there 
is a wider means than mineral character at command for co- 
ordinating water-formed rocks. Hence strata can be identified 
and correlated by discovering the physical conditions which limited, 
determuned, and changed their mineral characters, and changed the 
distribution of the fauna and flora of the given geographical area 
that they occupy. 
14. Nothing can be known of climatal conditions of the earth 
in past time, except from physical evidence. Such is the exist- 
ence of coal ; for, judging from the analogy of peat, there is 
strong reason for inferring that coal was formed under con- 
ditions of temperature not warmer than our English climate. 
15. The most important physical phenomena for the elucida- 
tion of past physical geography are the thickness of the deposit 
over a wide area, the number of beds of which it consists, the 
relative sizes and characters of the constituent particles at dif- 
ferent depths and in different districts, the amount and direc- 
tion of the false bedding &c., the exact vertical and geogra- 
phical position of fossils, &c. &c. 
16. Just as the phenomena of water-formed rocks all owe their 
existence directly or indirectly chiefly to the sun’s energy, so also 
do the phenomena interwoven with life. This has long been re- 
cognized by various eminent British and foreign physicists ; and, 
in 1854, Prof. Huxley, in his memoir on the method of paleon- 
tology, asserted that organisms were but manifestations of ap- 
plied physics and applied chemistry. Prof. Tyndall puts the 
generalizations of physicists in a few words: when speaking of 
the sun, it is remarked, “He rears....the whole vegetable 
world, and through it the animal; the lilies of the field are his 
workmanship, the verdure of the meadows, and the cattle upon 
a thousand hills. He forms the muscle, he urges the blood, he 
builds the brain. His fleetness is in the lion’s foot ; he springs 
in the panther, he soars in the eagle, he slides in the snake. 
He builds the forest and hews it down, the power which raised 
the tree and that.which wields the axe being one and the same.” 
