1834 ] 



Geology of the South of India. 



332 



en or nodular-basalt, composed of nuclei of basalt, usually of great 

 specific gravity, surrounded by concentric layers of a loose earthy 

 mass, resembling wacken, but without cohesion, which, on a super- 

 ficial view, conveys to the mind the idea of a fluid mass of earth, 

 having, in its descent from some higher spot, involved, in its course 

 all the rounded masses it encountered, and, subsequently, become 

 consolidated by drying. A very slight inspection is sufficient to de- 

 tect the true cause of this appearance, which is owing to the facili- 

 ties of decomposition of the outer crust, depending on difference of 

 structure and composition. In none of the conglomerates, or pud- 

 ding stones, do we observe any traces of this structure, and as it is 

 common to the most crystalline green-stone, porphyritic green-stone, 

 and those rocks usually denominated syenite, there can be little doubt 

 that it is owing to the developement of a peculiar concretionary struc- 

 ture by decomposition. In a small ravine, near the village of Sal- 

 minda, two thousand feet above the sea, I saw basalt of a perfectly 

 columnar structure, closely connected with a columnar mass formed 

 of concentric lamellse, enclosing a heavy and hard nucleus. Near 

 this ravine, I had also an opportunity of observing the gradual and 

 perfect passage of the columnar basalt into that which has been call- 

 ed stratified, from the parallelism of its planes ; the composition be- 

 ing identical, and, without doubt, cotemporaneous. These changes 

 and passages, from one rock into the other, are so frequent and va- 

 rious, as to render it impossible to refer the most of them to either 

 of the rocks I have abovementioned, as types. I shall, therefore, 

 proceed to describe those which are distinctly marked, and their ac- 

 companying minerals. In external appearance, the columnar and 

 semi-columnar basalt closely resembles that of the Giant's Cause- 

 way, possessing the same fracture, internal dark colour, and external 

 brown crust. It is equally compact and sonorous. It, however 

 contains, more frequently, crystals of oUvine, of basaltic hornblende, 

 and of carbonate of lime. The fusibility of each is the same. Per- 

 haps the basalt of the Gawilgerh range, more nearly resembles, in eve- 

 ry respect, that of the Pouce mountain in the Mauritius. This is, 

 however, of very little importance, since everybody vvho has travelled 

 much in trap countries, knows well what great changes in composi- 

 tion and structure occur even in continuous masses. Among the 

 minerals, calcedony, and the different species of zeolite, arc rarely 

 found in the columnar basalt, but they are of frequent occurrence in 

 that which is semi-columnar. The wEicken, or indurated clay, is as 

 various in character and composition, as the basalt, and, untorta- 



