<!!:<> !,<><; Y 



149 



WM still frequently appealed i<> in explana- 



tion oi oioyiaphir features which Jin- now i. 

 i.i .-.] t<> ! tin- work of epigene action. Such viewu 

 1 tin their upholders tin- name of Cataclys- 

 mi>ts in i 'ata-t : .i|.li i-t -. 1,\ i-ll's main idea that the 

 nt i> tin- type ii( .-ill [(receding ages, so far HH 

 ill.--.,- are n-xealed liv tlu- fossiliferons strata, has 

 I for his school the title of I'niformitarian. 

 Hut within recent yean* inanv of liis disciples have 

 !.! somewhat from tlie teaching of their 

 master, and maintain that the operations of 

 nature have been the same in kiml. hut not neces- 

 sarily in degree. The impulse given to the advance 

 of biological science by the publication of the Origin 

 /.- i I s.v.i i has also affected geology, and not 

 mi its palicontologiral vide alone. In the depart- 

 ini-iits of physical and stratigraphical geology one 

 ma\ note a larger and broader method of treatment 

 since the appearance of Darwin's famous work the 

 dominant tone in geological literature at present 

 being rather evolutional than nniformitarian in the 

 narrow sense. Another distinguishing feature of 

 geological science in our day is the great attention 

 paid to Petrography (q.v.), the study of which had 

 failen into comparative neglect in this country for 

 many years. Interest in it, however, was revived by 

 Dr Sorby, who showed how much might be learned 

 l'\ examining thin slices of rocks and minerals 

 under the microscope. The introduction of the 

 microscope into petrographical investigation has 

 thus opened up a wide ana novel field of inquiry, 

 f i oni the assiduous cultivation of which much may 

 be expected. 



It may be interesting to point out as shortly as 

 possible the order of development of the geological 

 sciences. Unquestionably the earliest to take 

 shape was Mineraloay a work on descriptive 

 mineralogy by Agricola having appeared in 1546. 

 In fact, several complete treatises nad been pub- 

 lished before the middle of the 18th century. 

 'ii/Hosy, or the study of the various rocks of 

 whic li the earth's crust is composed without special 

 reference to the mode of their arrangement, was 

 the kind of geology which chietiy occupied the 

 attention of the earliest investigators. The term 

 is now practically disused, and in its place we 

 " aye Petrography. When employed by modern 

 riters it has usually a wider signification (see 

 . N'OSY). Struct nml Geology, or the mode 

 in which rocks are built up in the earth's crust, 

 next began to come into prominence, and Dymtui- 

 t'-nf < lent i></t/, or the study of causes now in action 

 followed the system advocated by Hutton 

 Play fair being that which has gained general 

 ptance. Thereafter followed Experimental 

 '"/.'/. of which Hall was the father. Although 

 I'lM-n- had been made by Lehmann, Fuchsel, 

 ami Werner in the method of determining the 

 sin-region of strata and of grouping these in 

 chronological order, yet Historical or Stratigraph- 

 Geology can hardly be said to have existed 

 -rii-nce before the date of William Smith's 

 statical researches. Paleontology is of still more 

 n-.-1-nt origin, the names of Cuvier, Lamarck, and 

 Brongniart being conspicuous among its earliest 

 exponents. 



A brief outline may now be given of the various 

 MfNUtmente of geology, properly so called. 



1>VN AMICAI, GEOLOGY. The modern sstem of 



is l.a.-i'd on the principle that the past is 

 to In- interpreted through the present. In other 

 words, the geologist believes in the constancy of 

 nature, and that by studying the effects produced 

 by the action of her various agents in the present 

 he will l>e able to interpret the records of such 

 '> in the past. The study of such natural 

 iterations constitutes dynamical geology. 

 The various forms of energy from which geo- 



logical changes arise may l>e divided into two 

 vi/. &MKMMM iK'tinn and ejjiaene artinn. 



lli/l>nijnii- Actimt. -Under this head come the 

 changes which are induced by the internal heat of 

 the earth, those changes, namely, that are in pro- 

 gress In-neat h the earth's surface. In this category 

 are included volcanoes and volcanic action, volcanic 

 product-, ami the chemical and mechanical changes 

 which are superinduced in such products and upon 

 the lock-masses with which these come into contact 

 during volcanic eruptions (see VOLCANOES). Lava 

 (q.v.) and Tun" (q.v.) are studied as regards their 

 composition, texture, and structure, while the man- 

 ner in which these and other volcanic products are 

 built up is likewise investigated. All this is done 

 with a view to comparing such volcanic products 

 with similar crystal line and fragment al rocks which 

 occur in regions where volcanic action may have 

 li'-riiine quite extinct. Another most important 

 set of hypogene phenomena are movements of the 

 earth's crust. See EARTHQUAKE, UPHEAVAL AND 

 SUBSIDENCE, BEACHES, SUBMARINE FORESTS. 



Epigene action has reference to those operations 

 that affect mainly the superficial portion of the 

 earth's surface. The epigene agents are the atmo- 

 sphere, rain, brooks and rivers, ice, the sea, and 

 life. The effects of atmospheric action are seen in 

 the general disintegration of rocks, the formation 

 of Soil (q.v. ), and the accumulation of dust and sand 

 ( see DRIFT ). In the diffusion of life over the globe, 

 wind has also no doubt played in all ages an 

 important part. Rain, again, charged with the 

 carbonic acid, &c., which it absorbs from the atmo- 

 sphere and vegetable soil, acts chemically upon 

 rocks all of which are more or less permeable. 

 Much rock-disintegration is thus induced, the 

 ' weathered ' materials being dispersed or accumu- 

 lated locally by the mechanical action of the rain. 

 The chemical action of rain is not confined to the 

 surface of the ground, for much water niters down 

 through natural cracks, fissures, &e., and is thus 

 enabled to soak into the rocks at all depths. The 

 underground water which is not absorbed in the 

 interstitial pores of rocks rises eventually, and is 

 discharged at the surface as Springs (q.v.), which 

 are more or less impregnated with dissolved mineral 

 matter abstracted from below. These springs are 

 either cold or thermal, and constant or inter- 

 mittent. In some volcanic regions the water 

 comes to the surface in eruptive fountains (see 

 GEYSER). The destructive action of such under- 

 ground waters is seen in the excavation of caves, 

 tunnels, and other subterranean passages (see 

 CAVE), and in the production of Landslips (q.v.) 

 and rock-falls ; while their reproductive action is 

 familiarly illustrated by the formation of Stal- 

 actites and Stalagmites (q.v.), and the accumula- 

 tion of great masses and sheets of siliceous Sinter 

 and Calcareous Tufa (q.v.). Brooks and rivers 

 act as potent agents of change. By means of 

 the detritus which they sweep along or carry in 

 suspension, they rub, grind, and erode the rocks 

 over which they flow, and thus in time ravines 

 and valleys have been excavated. The eroded 

 materials are constantly travelling from higher to 

 lower levels until they come to rest in lakes or the 

 sea. Hence lakes and the sea in many places are 

 lieing gradually silted up the growth of Deltas (q.v.) 

 iM'ing one of the most notable evidences of epigene 

 action. The action of rain and running water is 

 greatly aided by frost, which is a powerful disin- 

 tegrator of rocks. Water freexes as well in the 

 minute pores of rocks as in the fissures by which 

 rocks are traversed, and thus when thaw ensues the 

 loosened grains and particles are ready to be carried 

 away by wind, rain, and melting snow ; while dis- 



{'ointed blocks, x.r. may fall asunder and topple 

 rom cliffs or roll down steep slopes. In regions of 



