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 he identified 

 and correlated by discovering the physical conditions which limited, 

 determined, 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; forejudging 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 ai'ea, 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 palseon- 

 tology, asserted that organisms w^ere 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 i-emarked, " 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 wliich wields the axe being; one and the same." 



