iv BOMB-FORMATION 47 



where they vary from two to five millimetres across. A vitreous 

 border, about an inch in breadth, forms the outer shell of the bomb 

 where it is in contact with the tuff. Some of the bombs are only 

 two or three inches apart ; and one of them shows evidence of 

 fracture, fragments of the outer vitreous shell lying imbedded in 

 disorder in the surrounding tuff. 



Before entering into more detail it may be at once observed 

 that the contiguity of some of the bombs to each other makes it-at- 

 first difficult to view them as having been formed in the manner 

 volcanic bombs are supposed to originate. Those who have seen 

 the huge bombs lying scattered about on the summit of Vulcano in 

 the Lipari Islands will appreciate the difficulty of imagining how 

 these bombs can occur in such a close arrangement without having 

 often shattered each other to fragments. However, Mr. Wittstock of 

 Mbaulailai in a letter to me describes even larger bombs that came 

 under his notice exposed on the surface in the Mbua district, their 

 outer crust when broken looking " like the slag of a blast-furnace." 



The bomb-rock is a semi-vitreous basaltic andesite. It displays 

 microporphyritic plagioclase in a ground-mass formed mainly of a 

 smoky, almost isotropic glass, in which numbers of felspar micro- 

 liths (*i mm.) are developed, the augite being but slightly differen- 

 tiated. Scattered about in the glass are little irregular patches, 

 or " lakelets," of residual magma composed of a yellowish feebly 

 refractive material that I cannot distinguish from palagonite. 



The ash, in which the bombs are imbedded, is a somewhat 

 friable hyalomelan-tuff composed of fragments of basic glass often 

 partially palagonitised, and usually 2 or 3 mm. in size. In it 

 occur pumiceous lapilli of the same material up to 2 centimetres 

 in diameter. The glass is markedly vacuolar, the cavities being 

 either filled with gas or with alteration-products. The vacuoles 

 are often drawn out into tubes, giving the glass a fibrillar appear- 

 ance. The numerous plagioclase phenocrysts inclosed in the glass 

 are much honeycombed and contain large inclosures of the glass, 

 both altered and unchanged. 



Although the line of contact is well defined in a hand-specimen, 

 the two rocks cannot be separated along the junction. In a thin 

 section, in which the union of the vitreous shell of the bomb with 

 the ash is well shown, there is no defined line of demarcation, the 

 non-vacuolar isotropic glass of the bomb being there broken up 

 into fragments, with the interspaces filled with the partially 

 palagonitised pumiceous ash. In the vitreous shell the felspar 

 microliths are much less developed both in size and number than 



