1839] 



Trap D:Jces in the Sienits of Jmboor, 289 



tion, I found it to be wholly composed of sienite, intersected by various 

 trap dykes of different dimensions. These dykes maintained, throughout 

 their whole extent, one uniform and parallel direction, being all at right 

 angles to the line of the road intersecting the main range, and conse- 

 quently parallel to the main range itself. 



The trap, of which these dykes are composed, consists of a matrix of 

 dark coloured granular hornblende, throughout which small grains of 

 mica are interspersed. 



On examining the trap at its lines of junction with the incUiding 

 sienite, decided marks of alteration are observable in the former, though 

 not in the latter — the hornblende becoming of a much lighter colour, 

 while the grains of mica become larger, and most distinctly developed. 

 Traversing the dykes, and invanahhj at right angles to the cooling sur- 

 faces of (he sienite, I found small cracks or fissures — a similar remark to 

 this, has been made on the comparatively recent ejections from certain 

 volcanoes, and more especially on those of mount Vesuvius, of which 

 Mr. Lyell remarks — " There is a tendency in all Vesuvian dykes to 

 divide into horizontal prisms, a phenomenon, in accordance Vvith the 



fonn^a flat rhomboidal prism, having an obtuse angle=about 130»and an acute angle- 

 about 50°; the want of a goniometer put it out of my power to do more than approxi- 

 mate to the side of the angles of the ihoniboid-cleavage, regular and single in a plane 

 at right angles to the axis of the crystal. Exposed to the action of undiluted muriatic acid 

 it deepened the colour of this to a dark and beautiful orange hue— to nitric acid it commu- 

 nit;ated a greenisk tinge, while in sulphuric acid no action whatever took place. From the 

 two first effects I was k-d to suspect the mineral to be one of the crystallized salts of iron, 

 and under this impression I added to each of the acid solutions, ammonia and oxalic acid, 

 expecting precipitation to follow. In this however I was disappointed; no such effect 

 being produced. The mineral strikingly resembles in external appearance the specu- 

 lar iron ore, but in comparison I found they differed in crystalline form, the latter being 

 octahedral w^hile the former as was before stated is rhomboidal. 



On examining the main body of the hill I found it composed of sienite, intersected by 

 numerous trap and porphyritic dykes— one of the latter I found capping the sienite, so as 

 to seem at first sight to be an overlaying rock, but on further examination its true nature 

 is easily detected and its limits observed. Much varictj- is observable in the composition, 

 of the dykes and veins. Sometimes the quartz of the porphyries disappears altogether, 

 ■when we have puie felspar left. Sometimes hornblende predominates, and some- 

 times nothing but quartz is seen. The felspar was often of a pinkish hue, and generally 

 so much decomposed on the surface as when pressed in the hand to crumble into dust— • 

 sometimes it became dendritic, the tree-like forms being due to small vesicular cavities, 

 presenting a very beautiful appearance. In the vicinities of the nullahs abundance of 

 water-worn masses of basalt, greenstone, claystone, &c. are met with, though as might 

 be anticipated they are more sparingly distributed at a distance from these. For further 

 particulars, see Dr. Benza's paper on the Geology of the route fi-om Madras to the Hills, 

 in No. XII of this JournaL 



