604 MINERAL COMPOSITION [Ch. XXV ITT 



Delesse to be trap impregnated with the red oxide of iron, and in 

 part reduced to kaolin. * When still more decomposed, they were 

 found to be clay colored by red ochre. As two of the lavas of the 

 Giant's Causeway are parted by a bed of lignite, it is not improbable 

 that the layers of laterite seen in the Antrim cliffs resulted from 

 atmospheric decomposition. In Madeira and the Canary Islands 

 streams of lava of subaerial origin are often divided by red bands of 

 laterite, probably ancient soils formed by the decomposition of the 

 surfaces of lava-currents, many of these soils having been colored red 

 in the atmosphere by oxide of iron, others burnt into a red brick by 

 the overflowing of heated lavas. These red bands are sometimes 

 prismatic, the small prisms being at right angles to the sheets of lava. 

 Eed clay or red marl, formed as above stated by the disintegration of 

 lava, scoriae, or tuff, has often accumulated to a great thickness in the 

 valleys of Madeira, being washed into them by alluvial action ; and 

 some of the thick beds of laterite in India may have had a similar 

 origin. In India, however, especially in the Deccan, the term " later- 

 ite " seems to have been used too vaguely. 



It would be tedious to enumerate all the varieties of trap and lava 

 which have been regarded by different observers as sufficiently abun- 

 dant to deserve distinct names, especially as each investigator is too 

 apt to exaggerate the importance of local varieties which happen to 

 prevail in districts best known to him. It will be useful, however, to 

 subjoin here, in the form of a glossary, an alphabetical list of the 

 names and synonyms most commonly in use, with brief explanations, 

 to which I have added a table of the analysis of the simple minerals 

 most abundant in the volcanic and hypogene rocks. 



Explanation of the Names, Synonyms, and Mineral Composition of 

 the more abundant Volcanic Bocks. 



Agglomerate. A coarse breccia, composed of fragments of rock, cast out of vol- 

 canic vents, for the most part angular and without any admixture of water- 

 worn stones. " Yolcanic conglomerates " may be applied to mixtures in 

 which water-worn stones occur. 



Aphanite. See Cornean. 



Amphibolite, or Hornblendic Rock, which see. 



Amygdaloid. A particular form of volcanic rock ; see p. 601. 



Augite Rock. A rock of the basaltic family, composed of felspar and augite. See 

 p. 598. 



Atjgitic-porphtry. Crystals of Labrador-felspar and of augite, in a green or dark 

 gray base. (Hose, Ann. des Mines, torn. 8, p. 22, 1835.) 



Basalt. An intimate mixture of felspar and augite with magnetic iron, olivine, &c. 



See p. 598. 

 Basanite. Name given by Alex. Brongniart to a rock, having a base of basalt, 



with more or less distinct crystals of augite disseminated through it. 



Claystone and Claystone-porphyry. An earthy and compact stone, usually of a 



