GEOLOGY 



3476 



GEOLOGY 



weight. Thus a block 74 m. thick 

 below a place in the Himalayas 

 that is 4 m. above sea level weighs 

 the same as a block equal in area 

 and 66 m. thick below the ocean 

 floor where it is 4 m. below sea 

 level. The greater thickness of the 

 Himalayan block would be com- 

 pensated by the greater density of 

 material in the sub-oceanic block. 



DYNAMIC GEOLOGY DENUDA- 

 TION, VOLCANOES, AND EARTH- 

 QUAKES. The surface of the litho- 

 sphere is constantly crumbling 

 under the attack of wind, air, and 

 water. The gases of the atmo- 

 sphere cause the decay and dis- 

 integration of rocks. Wind, rain, 

 and rivers carry away the decom- 

 posed material and expose fresh 

 layers to decay. This process is 

 known as denudation, and it is 

 steadily lowering the surface of 

 the land. The materials derived 

 from the wearing away of the land 

 are in time carried to the sea, and 

 there deposited as beds of sand or 

 clay which are formed into new 

 secondary rocks. 



The Work of Ice 



Denudation in cold countries 

 and on snow-clad mountains is 

 aided by the work of ice ; glaciers 

 flow from the mountain snow- 

 fields down the valleys, and carry 

 with them stones and earth which 

 are deposited when the glacier 

 melts in ridges known as moraines. 

 The geological work of ice is at 

 present restricted mainly to moun- 

 tains and to low levels in the Polar 

 regions ; but in former times large 

 tracts of country which are now ice 

 free were once covered by glaciers. 

 The British Isles, for example, were 

 in comparatively recent geological 

 times covered by a sea of ice, which 

 was formed upon the mountains of 

 Scotland, the N. of England, and 

 Wales ; it flooded all the northern 

 part of the country, and deposited 

 wide sheets of boulder clay. The 

 remains of various older glacia- 

 tions have been found even in 

 tropical regions. One of the most 

 famous is known from isolated 

 glacial deposits in India, Australia, 

 S. Africa, and S. America, which 

 were formed by extensive glaciers 

 at the time when the growth of 

 luxuriant vegetation in Europe and 

 the U.S.A. was producing the 

 materials for their chief deposits 

 of coal. 



A second group of dynamic pro- 

 cesses depends on subterranean 

 actions, of which the most striking 

 are those connected with volcanoes 

 and earthquakes. A volcano is a 

 pipe up which molten rock is forced 

 to the surface by . its included 

 gases or steam, or by pressure due 

 to earth movements. An earth- 

 quake is a sudden violent move- 



ment of the ground ; it may be due 

 to a volcanic explosion, which may 

 be so powerful as to shake the 

 whole earth, or it may be due to 

 the slipping of a mass of sediment 

 down a steep slope especially be- 

 neath the sea, or it may be due to 

 the uplift or subsidence of part 

 of the earth's crust by faulting or 

 folding. From the locality where 

 the initial movement takes place 

 a shock passes outward in all 

 directions, and is felt as an earth- 

 quake. 



HISTORICAL GEOLOGY. Histori- 

 cal geology depends on two main 

 principles. The first is that of 

 superposition. As the secondary 

 rocks of the crust are laid down 

 in layers, one on top of the other, 

 it follows that the lowest rock 

 in a series is the oldest and 

 the uppermost is the youngest. 

 Superposition is therefore alone 

 usually a safe guide to the relative 

 ages of adjacent rocks. It cannot, 

 however, be implicitly trusted, 

 since series of tilted rocks are in- 

 verted by earth movements, hence 

 a rock may lie upon one that was 

 originally above it. Superposition 

 is also inapplicable to the com- 

 parison of rocks in distant parts 

 of the world. The final deter- 

 mination of the age of rocks de- 

 pends on the second method the 

 use of fossils. Fossils are the re- 

 mains or impressions in a rock of 

 animals or plants that lived during 

 its formation. The early animals 

 and plants were primitive in cha- 

 racter, and, in accordance with the 

 theory of evolution, there has been 

 a gradual change from them to the 

 more complex organisms of later 

 times. Fossils, therefore, can be 

 used like medals or coins to deter- 

 mine the dates of ancient ruins. 

 Geological Periods 



The discovery by William Smith 

 (1769-1839) that fossils could be 

 used in this way gave him his 

 title of Father of Geology. He 

 showed that fossils are " the 

 medals of creation," for each geo- 

 logical period was characterised 

 by particular types of life. For 

 example, the graptolites lived in 

 the world during the three older 

 subdivisions of the Palaeozoic 

 period. Any rock containing a 

 graptolite, in whatever part of 

 the earth it may be found, may be 

 <i&fely identified as Lower Palaeo- 

 zoic. The main time scale used in 

 geology is based upon the succes- 

 sion of life. Sir Charles Lyell 

 termed the group of rocks contain- 

 ing the oldest known fossils the 

 Palaeozoic or period of ancient life. 

 It was succeeded by the Mesozoic 

 or period of middle life, and that in 

 turn by the Kainozoic or period of 

 recent life. Before the Palaeozoic 



there are two great groups of rock 

 in which no definite fossils have 

 been found. The older of these two 

 groups is the Eozoic, its rocks are 

 igneous or metamorphic, and un- 

 fossiliferous. It was succeeded by 

 the Archaeozoic, a period mainly 

 composed of unaltered secondary 

 rocks which are earlier than the 

 Palaeozoic, and contain some 

 obscure fossil remains. The Palaeo- 

 zoic, Mesoazoic and Kainozoic have 

 been also called the Primary, 

 Secondary, and Tertiary respec- 

 tively. Of these terms the two 

 former have been abandoned, but 

 the term Tertiary is still often Vied 

 for Kainozoic. 



Palaeozoic Fossils 



The pre-Palaeozoic rocks con- 

 tain only obscure or indirect traces 

 of life, which probably originated 

 by the formation and modification 

 of carbohydrates under the special 

 atmospheric and climatic condi- 

 tions which must have existed at 

 one stage of the early earth. The 

 first organisms must have been 

 small, and would have had no hard 

 parts which could leave traces in 

 the rocks. Fossils begin suddenly 

 in great variety and abundance 

 with the Palaeozoic, their abrupt 

 beginning indicates that at one 

 period many groups of soft-bodied 

 creatures simultaneously deve- 

 loped shells, and could thus be 

 preserved as fossils. This rapid 

 spread of shell formation may have 

 been either as a protection against 

 a group of animals which had 

 become carnivorous, or owing to 

 some change in the quality of sea 

 water by which shells were ren- 

 dered possible. 



From the beginning of the Palae- 

 ozoic age fossils have been 

 plentiful ; in the lower Palae- 

 ozoic systems there were no 

 back-boned animals, of which the 

 fish occur as early as the Silurian. 

 Reptiles appeared in the upper 

 Palaeozoic, when the rank vege- 

 tation of the Carboniferous and 

 Permian produced the world's 

 chief deposits of coal. The Meso- 

 zoic period, especially represented 

 in England by the oolitic limestone 

 and the chalk, was the age of rep- 

 tiles ; but during it birds and 

 mammals both made their first 

 appearance. Mammals became 

 supreme in the Kainozoic, the end 

 of which was marked by the ad- 

 vent of man. The date of the 

 oldest vestige of man is the sub- 

 ject of active research and con- 

 troversy. The crags of East 

 Anglia (Upper Pliocene in age) 

 have yielded many chipped flints 

 which are regarded by some au- 

 thorities as wrought by man : if 

 so, they are the oldest of human 

 implements. The most primitive 



