chap. v. Fluidity of different Lavas. .119 



angular vesicles, 1 have possessed little fluidity ; but we 

 see that the case has been very different at Albemarle 

 Island. The degree of fluidity in different lavas, does 

 not seem to correspond with any apparent corresponding 

 amount of difference in their composition : at Chatham 

 Island, some streams, containing much glassy albite 

 and some olivine, are so rugged, that they may be com- 

 pared to a sea frozen during a storm ; whilst the great 

 stream at Albemarle Island is almost as smooth as a 

 lake when ruffled by a breeze. At James Island, black 

 basaltic lava, abounding with small grains of olivine, 

 presents an intermediate degree of roughness ; its sur- 

 face being glossy, and the detached fragments resem- 

 bling in a very singular manner, folds of drapery, cables, 

 and pieces of the bark of trees. 2 



Craters of tuff. — About a mile southward of Banks' 

 Cove, there is a fine elliptic crater, about 500 feet in 

 depth, and three quarters of a mile in diameter. Its 

 bottom is occupied by a lake of brine, out of which 

 some little crateriform hills of tuff rise. The lower 

 beds are formed of compact tuff, appearing like a 



1 The irregular and angular form of the vesicles is probably 

 caused by the unequal yielding of a mass composed, in almost equal 

 proportion, of solid crystals and of a viscid base. It certainly seems 

 a general circumstance, as might have been expected, that in lava, 

 which has possessed a high degree of fluidity, as well as an even- 

 sized grain, the vesicles are internally smooth and spherical. 



2 A specimen of basaltic lava, with a few small broken crystals 

 of albite, given me by one of the officers, is perhaps worthy of de- 

 scription. It consists of cylindrical ramifications, some of which are 

 only the twentieth of an inch in diameter, and are drawn out into 

 the sharpest points. The mass has not been formed like a stalactite, 

 for the points terminate both upwards and downwards. Globules, 

 only the fortieth of an inch in diameter, have dropped from some of 

 the points, and adhere to the adjoining branches. The lava is vesi- 

 cular, but the vesicles never reach the surface of the branches, 

 which are smooth and glossy. As it is generally supposed that vesi- 

 cles are always elongated in the direction of the movement of the 

 fluid mass, I may observe, that in these cylindrical branches, which 

 vary from a quarter to only the twentieth of an inch in diameter, 

 every air-cell is spherical. 



