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•of many years their courses remain apparently unmodified. In less temperate lands, however, erosion often proceeds apace; watercourses are deepened and widened in an incredibly short time. During a tropical storm of rain as much erosion of soil and rock and transport of material are effected within a limited drainage-area as would tax a British river with all its tributaries to accomplish in a year or a number of years. Now these islands of ours have experienced many vicissitudes—tropical, subtropical, and arctic conditions have formerly obtained here— and we need not doubt, therefore, that the present rate of denudation has often been exceeded in the past. When streams and rivers began their work of erosion in the British area, it is probable that the climatic conditions were more favourable for that work than is now the case. In a word, although the wrork performed by geological agents of change has been the same in kind, it has necessarily varied in degree from time to time. The present rate of erosion in Britain, therefore, can be no infallible index to that of the past. But however rapidly denudation may have proceeded in former ages, the shaping out of our hills and valleys, even under the most favourable conditions, must have been a slow process. Nevertheless recent investigations leave little room for doubting that the time required for the evolution of all the multitudinous forms assumed by the land has been exaggerated. The tale told by our relict mountains and erosion valleys^does not support the claim for unnumbc
TABLE OF GEOLOGICAL SYSTEMS, AND THEIR PRINCIPAL
QUATERNARY Or (
Danian (not represented in England).
Senonian (Upper Chalk with Flints).
Turonian (Middle Chalk).
Cenomanian(LowerChalk and Upper Greensand).
,, .■ ■ / (Lower Greensand and Wealden Urgoman . > *
Bajocian (Inferior Oblite)
Toarcian (Upper Lias).
Liasian (Middle and Lower Lias
Sinemurian (Lower Lias in part)
Muschelkalk (not represented in England)
White Jura or Malm of Germany.
. ) Brown Jura or Dog) ) ger of Germany.
Black Jura or
[note.—The names of the subdivisions of the various systems given in this table are those generally accepted. Many, it will be seen, are of English origin ; others are foreign. Beside some of the latter the English equivalents (which are still current) are placed within parenthesis. A few German equivalents are given because reference is made to them in the text.]
Abrasion: the operation of wearing away by aqueous or glacial action.
Acid igneous rocks: rocks which contain a large percentage of silica to a small percentage of bases.
Agglomerate: volcanic fragmental rock, consisting of large angular, subangular, and roughly rounded blocks, confusedly huddled together.
Alluvium : a deposit resulting from the action of rivers or of tidal currents.
Amygdaloidal (Gr. amygdalon, an almond; eidos, an appearance): applied to igneous rocks containing vesicular cavities which have become filled, or partially filled, with subsequently introduced minerals. The cavities are frequently almond-shaped; the mineral kernels are termed amygdules.
Anticline (Gr. anti, against ; klino, I lean): a geological structure in which strata are inclined in opposite directions from a common axis; i. e., in a roof-like form. When its axis is vertical, an anticline is symmetrical; in an unsymmetricel anticline the axis is inclined.
Archsean : synonymous with Pre-Cambrian. See Table of Geological Systems.
Arenaceous: applied to strata which are largely or wholly composed of sand.
Argillaceous: applied to rocks composed of clay, or in which a notable proportion of clay is present.
Ash, volcanic: the finest-grained materials ejected during volcanic eruptions.
Basalt: a dark, hemicrystalline, basic igneous rock.
Base-level of Erosion: that level to which all lands tend to be reduced by
denudation. A land bast-levelled is usually very slightly above the sea-level,
and shows a gently undulating or approximately flat surface. Basic igneous rocks: rocks which contain a large percentage of bases to a
low percentage of silicic acid. Beaches, raised: former sea-margins ; sometimes appear as terraces of gravel,
sand, etc., sometimes as shelves cut in solid rock ; occur at all levels, from
a few feet up to several hundred yards above the sea.