chap. v. Segment of a small Crater. 125 



and three inches below, of red scoriaceous rock (which 

 is the case with all the streams), making altogether a 

 thickness of fourteen inches : this thickness was pre- 

 served quite uniformly along the entire length of the 

 section. A second stream wa.s only eight inches thick, 

 including both the upper and lower scoriaceous surfaces. 

 Until examining this section, I had not thought it 

 possible that lava could have flowed in such uniformly 

 thin sheets over a surface far from smooth. These 

 little streams closely resemble in composition that 

 great deluge of lava at Albemarle Island, which like- 

 wise must have possessed a high degree of fluidity. 



Pseudo-extraneous, ejected fragments. — In the 

 lava and in the scorise of this little crater, I found 

 several fragments, which, from their angular form, 

 their granular structure, their freedom from air-cells, 

 their brittle and burnt condition, closely resembled 

 those fragments of primary rocks which are occasionally 

 ejected, as at Ascension, from volcanos. These frag- 

 ments consist of glassy albite, much mackled, and with 

 very imperfect cleavages, mingled with semi-rounded 

 grains, having tarnished, glossy surfaces, of a steel-blue 

 mineral. The crystals of albite are coated by a red 

 oxide of iron, appearing like a residual substance ; and 

 their cleavage-planes also are sometimes separated by 

 excessively fine layers of this oxide, giving to the 

 crystals the appearance of being ruled like a glass 

 micrometer. There was no quartz. The steel-blue 

 mineral, which is abundant in the pinnacle, but which 

 disappears in the streams derived from the pinnacle, 

 has a fused appearance, and rarely presents even a 

 trace of cleavage ; I obtained, however, one measure- 

 ment, which proved that it was augite ; and in one 

 other fragment, which differed from the others, in 

 being slightly cellular, and in gradually blending into 



