50 THE VOYAGE OF H.M.S. CHALLENGER. 



rock resembling agate, and sometimes passing into pitclistone. A white substance 

 resembling pumiceous cinders fills the interstices between the nodules of obsidian. 

 Finally, the substance, which previously was spread through the rock, becomes an angulo- 

 concretionary mass of obsidian of a pale grey colour, and often traversed by coloured 

 bands parallel to those of the enclosing rock. Darwin then describes the rocks which 

 usually occur as stages in the transition to obsidian, and treats in a specially detailed 

 manner of the linear arrangement of spherulites. He explains the nodular form of some 

 specimens of obsidian by viewing them as concretionary masses like spherulites. After 

 discussing the chemical composition of these obsidian spherules, as known at that time, 

 he attributes the nodular and spherulitic forms to a process of segregation in the fused 

 mass which led to the separation of the parts richest in silica. He pointed out the 

 similarity between the phenomena exhibited by volcanic glasses and the devitrification of 

 artificial glass. Finally, Darwin compares his observations on the obsidian of Ascension 

 with those of Beudant in Hungary, of Von Humboldt in Mexico and Peru, and with 

 the descriptions by other geologists who had brought analogous facts to light in various 

 volcanic regions. 



Having recalled Darwin's work on the obsidians of Ascension, we shall proceed 

 to give a lithological description of the specimens of this rock which we have 

 examined ; these came from Green Mountain. When the specimens are not weathered, 

 they present all the ordinary characters of obsidian, being black, vitreous, with a 

 brilliant lustre, conchoidal fracture, and transparent at the edges. They are often 

 cracked, the margins of the fissures appearing as white fines, and sometimes they are 

 slightly scoriaceous with a more irregular fracture. When weathered the surface becomes 

 greyish and earthy in appearance, and when the rocks decompose they sometimes 

 assume a waxy lustre like retinite. They are often veined with greenish or greyish 

 lines, and at other times finely zonary ; in this case they are seen by the naked eye to 

 be furrowed with little undulating parallel veins that stand out grey against the black 

 background of the rock. When zonary obsidian weathers, its conchoidal fracture is 

 obscured, and the fragments break along the zones. The only macroscopic constituent 

 is sanidine, which stands out from the ground-mass in vitreous grains, sometimes of 

 considerable size. Microscopic examination shows that all the obsidians of Ascension 

 are made up of a light brown vitreous matter, the colour of which becomes darker in 

 bands where microliths accumulate. Microporphyritic structure is somewhat rare, and 

 when seen it is always brought about by sections of sanidine. The glass is, however, 

 never homogeneous ; besides the elongated vesicles, often arranged in bands, there 

 are little lamellae of sanidine and minute prisms of augite 1 scattered through the base. 



1 Darwin points out (p. 55 he. cit.) that Miller determined as augite some fine green needles in the rocks of 

 Ascension associated with obsidian. The rocks yielding these microliths also contain, according to Miller, crystals of 

 quartz, which he measured and found possessed of the faces P, z, m, without a trace of r. 



