GLOSSARY 
Of Geological and other Scientific Terms used in this Work. 
Several of the Author's friends, who had read the first and second volumes of 
the Principles of Geology, having met with difficulties from their previous tin- 
acquaintance with the technical terms used in Geology and Natural History, 
suggested to him that a Glossary of those words would render his work much 
more accessible to general readers. The Author willingly complied with this sug- 
gestion, but finding that his own familiarity with the subject made him not a very 
competent judge of the terms that required explanation, he applied to the friends 
above alluded to for their assistance, and from lists of words supplied by them, the 
following Glossary has been constructed. It will be obvious to men of science, 
that in order to attain the object in view, it was necessary to employ illustrations 
and language as familiar as possible to the general reader. 
Acephalous. The Acephala are that division of molluscous animals 
which, like the oyster and scallop, are without heads. The class 
Acephala of Cuvier comprehends many genera of bivalve shells, 
and a few genera of mollusca which are devoid of shells. Etym., 
a, a, without, and KstyaXrj, cephale, the head. 
Alg^;. An order or division of the cryptogamic class of plants. 
The whole of the sea-weeds are comprehended under this divi- 
sion, and the application of the term in this work is to marine 
plants. Etym., Alga, sea-weed. 
Alum-stone, Alumen, Aluminous. Alum is the base of pure clay, 
and strata of clay are often met with containing much iron-pyrites. 
When the latter substance decomposes, sulphuric acid is pro- 
duced, which unites with the aluminous earth of the clay to 
form sulphate of alumine, or common alum. Where manu- 
factories are established for obtaining the alum, the indurated 
beds of clay employed are called Alum-stone. 
Alluvial. The adjective of alluvium, which see. 
Alluvion. Synonymous with alluvium, which see. 
Alluvium. Earth, sand, gravel, stones, and other transported 
matter which has been washed away and thrown down by rivers, 
floods, or other causes, upon land not permanently submerged 
beneath the waters of lakes or seas. Etym., Alluo, to wash 
upon. For a further explanation of the term, as used in this 
work, see vol. ii. chap, xiv., and vol. iii. p. 145. 
