xiv The Record of the Rocks 265 



Coal, for example, is the residue of partially decomposed 

 vegetation. 



344. Interpretation of Fossils. As a general rule it 

 is assumed that the creatures whose remains occur in the 

 rocks were similar in their habits to those now living, and 

 were in an equal degree dependent on the climate. Rocks 

 formed of the sediment of lakes and rivers may, by the 

 greater abundance of land creatures amongst their fossils, 

 be distinguished from those composed of marine deposits. 

 These inferences are often confirmed by the nature of the 

 rocks themselves, the fine mud of estuaries naturally yield- 

 ing a shale, while the pebbles of an exposed seashore are 

 compacted into a conglomerate. Rocks containing the 

 remains of the same species of creatures have evidently 

 been formed under similar physical conditions, and possibly 

 at the same time ; hence they are said to belong to the 

 same geological horizon. 



345. Divisions of Sedimentary Rocks. There is so 

 much scope for individual opinion in interpreting the record 

 of the rocks that no minute classification of them meets the 

 approval of all competent geologists, but a few compre- 

 hensive divisions are generally accepted. The most ancient 

 sedimentary rocks are allowed to be those containing fossils 

 exclusively of the simplest form of life. The variety and 

 complexity of the organisms found, usually increase as the 

 more recent strata are approached. The greatest thickness 

 of a bed of sedimentary rock may in some cases give a 

 rough measure of the shortest time it could have taken 

 in formation, but all attempts at fixing a definite geological 

 chronology have as yet been unsatisfactory. The great 

 divisions of rocks and their more important subdivisions 

 are given below in the order of antiquity, and some typical 

 forms of life are mentioned. 



QUATERNARY 



RECENT Now forming. 



PLEISTOCENE All modern plants and animals. Man. 



TERTIARY 



PLIOCENE Most modern plants. Elephant, Ox. 



