584 JOURNAL, BOMBAY NATURAL HIST. SOCIETY, Vol. XXVII. 



we assume that the upper trap in Bombay had its origin in the main- 

 land, since no local foci of eruption have been met with, and that it flowed 

 in a westerly direction, the more fluid portions would travel faster and cool 

 more rapidly, thus producing the basaltic trap of WorU. That it is merely a 

 question of fluidity and cooling would seem to be clear from the appearance 

 of an undivided mass of rock 105 ft. in thickness lying above the sedimentary 

 deposit at Sewri , a rock which is probably identical with the flow of which the 

 WorU basalt is the western outpost. Where it rests on the sedimentary bed 

 it is ashy grey and earthy, but gradually passes into a dark crystaUine rock at 

 the top, but less so than that at WorH. That such must have been the condi- 

 tion of the trap at the latter place at a very remote period is proved by bould- 

 ers of ashy grey and basaltic trap found embedded in the mass of moorum 

 resulting from the disintigration of the trap. It is a very noticeable fact thaj 

 the whole depth of rock on the eastern side of the hill has decomposed into 

 moorum by concentric disintigration of the trap, leaving rounded cores of vari- 

 ous sizes. The appearance of the more highly weathered of these cores is that 

 of rusted iron balls. The larger and less weathered ones still show the typical 

 trap characteristics. On the completion of the weathering process ths moorum, 

 it would appear, gets pressed into sheets and can easily be split into thin laminae. 

 It is not possible to say what are the determining causes which lead trap to 

 weather into balls ; the probability is that the lava in cooling assumes a definite 

 hexahedral form. The transition from this form to a ball in weathering is not 

 distant. The moorum which is yellowish brown in its lower reaches passes 

 gradually as it approaches the summit into the best red earth available in 

 Bombay. The quarried face of the moorum presents remarkably clear strati- 

 fications. The western face of the hillock on which stands the tomb 

 of Mama Hajiani is also decomposing into moorum but in a lesser degree 

 leaving in many instances masses of solid rock poised on columns of moorum 

 cut out by the action of the waves. It is a queer fact that the lower layers of 

 the rock are weathering earlier and faster than the vipper ones. 



The imweathered rock presents an appearance unlike that of any other trap 

 in Bombay. A freshly broken specimen shows a dark, fine grained crystalline 

 surface with sharp edges. When struck it sounds with a clear metallic ring. 

 In hardness and colour it ranks next to the " Lydian stone " of Antop Hill, the 

 hardest stone in Bombay. When seen in the mass it shows on the surface a 

 network of Lines, probably occasioned by cooling, which form more or less 

 hexagonal figures. These are best seen at Love Grove near the Sewage 

 outfall. There it is possible to see practically perfect hexagons occasionally 

 standing out as columns in the manner of true basalt. These lines penetrate 

 deep into the mass of the rock as seen at Nepean Sea Road, Malabar Hill, and 

 are filled in with quartz or zeolite veins. On both sides of these veins the rock 

 has become greenish brown to a slight extent, a sign of incipient weathering. 

 So far no cavities or geodes containing quartz or other minerals have been met 

 with. 



It would be interesting to investigate why this basaltic trap, which appar- 

 ently is harder than the other varieties, has weathered to such an extent as to 

 produce a bed of moorum over 100 ft. in thickness. The other hiUs in Bombay 

 are covered with a comparatively thin layer of soil. Mr. Wynne, however, is 

 of opinion that the basaltic trap is less susceptible to weathering, for, in 

 speaking of the basaltic trap of Malabar Hfll, which is identical with the Worli 

 trap, he says : " This is an extremely hard dark variety of bedded trap ; it 

 marks glass, yields but little, and in a peculiar manner, to atmospheric action ; 

 in some places the only effect produced being a slight superficial oxidation of 

 its combined iron, in others it is traversed by strong joints, between which large 

 spaces have been formed, and most of the remaining angles, owing to a rudely 

 developed concretionary structure, have been partially rounded off. " 



