10 Physko-Geognostic Sketch of the Island of 'Qahu, 



parietes are often lined, as in No. 28, with a greenish matter. In 

 all these specimens, with the exception of No. 17, the cavities pre- 

 sent the idea of having been formed by the extrication of gaseous 

 matter, in a viscid mass, not of cavities which had been once 

 filled with crystalline matter that had subsequently disappeared. 

 In some, the approach to the scoriform character is very close, 

 in others, scoriform, and nearly compact portions are placed in 

 close juxta-position in the same hand specimen, as in No. 8, 9, 

 and 10, which shews upon how weak a foundation is built the 

 determination of the age of these rocks, by their degree of com- 

 pactness simply. Sometimes a rock will be in one portion of the 

 mass compact and basaltiform, while in others closely adjacent, 

 it will be filled with empty amygdaloidal-shaped cavities. 



Cellular Lava. — The third series of the volcanic rock are the 

 cellular or scoriform, comprising specimens, Nos. 24, 5, 25, 44, 

 27, 26, 8, 45, 43, 6, 41, 4, 7, 37, 39, 47, 38. The cells vary in 

 number from a few scattered amygdaloidal-shaped cavities, till the 

 whole mass resembles closely in porosity some kinds of pumice. In 

 some the roughness and unyielding texture of the mass, and the 

 size and regular form of the cells, give to the whole a slaggy 

 appearance. In many and in all the more cellular varieties, the 

 cavities are empty ; in others they are filled with crystals of oli- 

 vine, which are frequently undergoing decomposition, leaving 

 the cavities partially empty. Angular crystals of mica and fel- 

 spar frequently give to the rocks a porphyritic character ; these 

 crystals are, however, always of small size. The mica is fre- 

 quently very abundant, forming, with its shining hexagonal 

 plates, the only imbedded mineral. This is well marked in the 

 slaggy lava from the coast at Waimea. In some of the more 

 cellular varieties, the laminae of mica become more elongated, and 

 are laid along the interstices of the cells, which they never cross. 

 The felspar is white, and dull in lustre. In the lavas of one 

 of the ridges descending from Mouna Raala, it forms a very 

 abundant ingredient, giving to the rock a spotted appearance. 

 This felspar diminishes sometimes so much in the size, whilst 

 the number of its crystals increased, that it forms part of the 

 mass of the rock, which, in such cases, is less cellular, but more 

 compact, rough, and trachytic in its fracture. The most com- 

 mon colour of the basis of these cellular lavas is ash-grey. Oc- 

 casionally the tint becomes reddish, distinct from that of the 



