150 



GEOLOGY 



perennial snow-fields the avalanche and the glacier 

 likewise act as important denuders of the surface, 

 and transporters of rock-debris from higher to lower 

 levels (see AVALANCHES, GLACIERS, BOULDER- 

 CLAY, &c.). Again, in certain latitudes lake and 

 river ice are conspicuous agents of change acting 

 especially as rafts for the transport of stones and 

 deoris (see ANCHOR-ICE). Thus the whole surface 

 of the land from the highest mountains down to 

 the sea is being gradually degraded or lowered by 

 the combined action or many epigene agents. 

 There is a continual and universal disintegration 

 of rocks going on, and a no less continual transport 

 of material and building up of this into new forma- 

 tions. Alluvial flats and terraces, deltas, &c. may 

 be cited as prominent examples of the sedimentary 

 series of modern accumulations, while the chemical 

 series is well represented by the calcareous forma- 

 tions of springs and brooks, and the precipitations 

 of common salt, sulphate of lime, &c., which are 

 taking place in saline lakes (see LAKE). 



The sea as a geological agent acts in three ways : 

 it erodes rocks, and transports and accumulates 

 sediment. The work of erosion is confined for the 

 most part to that marginal belt within which 

 waves and breakers work. These by means of the 

 shore-detritus batter and undermine cliffs, and 

 cause them gradually to recede, and hence the sea 

 may be said to act like a great horizontal saw. 

 The materials brought down by rivers or detached 

 from the shore by the action of the sea itself 

 are distributed by currents over the sea-floor, the 

 coarser detritus gathering in shallow water, while 

 the finer sediment is swept out to greater depths 

 and spread over wider areas. Such terrigenous 

 materials extend outwards from the shore to a 

 distance of 60 to 300 miles, and to depths of 2000 

 feet or more. They are confined, therefore, to 

 a comparatively narrow belt of the sea-bottom. 

 Over the abysmal depths of the sea, the only 

 accumulations in progress are organic ooze and a 

 peculiar red clay which is believed to be the result 

 of the chemical action of sea- water on products of 

 volcanic origin (see ABYSMAL ACCUMULATIONS). 

 Now and again, stones and debris may be carried 

 out to sea by icebergs and dropped beyond the zone 

 of terrigenous sedimentation. Similarly, rock-frag- 

 ments entangled in the roots of trees or buoyed up 

 by seaweeds may now and again come to rest in 

 abysmal regions. Reference has been made to the 

 geological action of the ice of lakes and rivers, but 

 the icebergs and ice-rafts of high latitudes must 

 not be omitted. Much rock-debris is distributed 

 by such agencies over the sea-bottom, detached 

 fragments of the 'Ice-foot' (see under ICE, Vol. VI. 

 page 59) being the most notable carriers of stones. 



The action of plants and animals is not ignored 

 by geologists. Plants aid in the disintegration 

 and rupture of rocks by means of their roots and 

 the organic acids derived from them during decay. 

 Rocks are drilled and bored by some kinds of 

 marine molluscs, annelids, echini, and sponges, 

 and are thus weakened and more readily yield to the 

 action of waves and breakers. Burrowing animals 

 also bring about changes, the common earthworm 

 being an efficacious agent in the formation of 

 soil (see EARTHWORM). Plants occasionally act 

 as conservative agents, as in the fixing of blown 

 sands (see DUNES), and in protecting the banks of 

 lakes and rivers. Again, forests, by equalising and 

 regulating the flow of the water of precipitation, 

 prevent the destruction of soils and subsoils by 

 torrential action. In some regions also the rocks 

 along a seashore are partially protected from the 

 waves by seaweed, sponges, zoophytes, and gre- 

 garious molluscs. Amongst formations of organic 

 origin may be mentioned soil (in part), peat-bogs, 

 morasses, .mangrove-swamps, bog-iron ore, &c. 



Some calcareous algae also form considerable beds, 

 as among the reefs of the Florida seas ; while certain 

 marsh -loving and fresh-water plants have the power 

 of abstracting carbonate of lime from water and 

 encrusting themselves therewith. Thick masses of 

 calc-tufa nave originated in this way. The organic 

 oozes of the deep seas are good examples of de- 

 posits formed or the exuvise of minute pelagic 

 organisms; and the great coral-reefs (see CORAL) 

 of the warmer oceans are still further evidence of 

 the importance of life in the production of new 

 formations. Such are some of the accumulations 

 which are almost wholly composed of organic debris ; 

 but animals and plants contribute to the growth 

 of many other deposits. The marine terrigenous 

 formations are charged more or less abundantly 

 with the relics of animal and plant life ; nor are 

 similar remains wanting in the alluvial deposits of 

 rivers and lakes. 



PETROLOGY. From the study of causes now in 

 action the geologist learns that many of the 

 rocks, with which every one, whether observant or 

 not, necessarily makes some acquaintance, are of 

 the same character as epigene and hypogene pro- 

 ducts. For a particular account of the rocks them- 

 selves, PETROGRAPHY and the articles therein cited 

 may be consulted ; here all that can be attempted is 

 to point out very briefly how far a knowledge of 

 formations now in progress enables us to explain 

 the nature and origin of rocks. 



(1) Igneous Rocks. In Great Britain and other 

 countries where at present there is no volcanic action 

 we meet with various glassy rocks, such as pitch- 

 stone and obsidian, with semi-crystalline rocks, as 

 trachyte, phonolite, liparite, andesite, basalt, &c., 

 with crystalline rocks, such as certain dolerites, 

 and with fragmental rocks, like tuff and agglo- 

 merate, which in every essential particular resemble 

 the products of modern volcanoes. But, as might 

 have been expected, the older igneous rocks are 

 often more or less altered, such alteration having 

 been superinduced by the chemical action of percolat- 

 ing waters, by pressure, by crushing, or by these 

 and other causes combined. There is a class of 

 crystalline rocks, however, which, although they 

 consist of the same mineral ingredients as occur in 

 many igneous rocks, yet differ so materially in 

 character from lavas that geologists are warranted 

 in believing that they could not have been con- 

 solidated at or near the surface of the earth. This 

 class is represented by such rocks as granite, 

 syenite, gabbro, and certain diorites, dolerites, 

 quartz-porphyries, &c. A study of these rocks under 

 the microscope and in the field as rock-masses leads 

 to the belief that they are indeed of igneous origin, 

 but have cooled and consolidated at some depth in 

 the earth's crust, their appearance at the surface 

 being due to subsequent denudation. Thus two 

 classes of igneous rocks are recognised viz. vol- 

 canic or superficial, and plutonic or deep-seated. 



(2) Derivative Bocks. Under this head are 

 included all the products of epigene action. They 

 are termed derivative inasmuch as most of them 

 are composed of materials which have been derived 

 from pre-existing rocks by the chemical or mechan- 

 ical action of epigene agents, while others are 

 made up of organic debris. They may be roughly 

 classified as follows : 



Mechanically-formed Rocks. These consist of 

 fragmental materials. They are granular non- 

 crystalline aggregates, the constituent ingredients 

 of which may be angular or rounded in form, and 

 may or may not be arranged in layers. They 

 consist of (a) Eolianor Aerial rocks, such as blown 

 sand (dunes) and dust- deposits. The products of 

 the ' weathering ' action of the atmosphere, such as 

 rock-debris (breccia), certain clays, &c., are also 

 in part of eolian origin. ( b ) Sedimentary rocks, 



