1845.] across the Peninsula of Southern India. 411 



nated fissile shale of various shades of brown, chocolate, red, and yel- 

 low passing into a pure white. These rocks have a distinctly jointed 

 structure : the joints are nearly vertical running in a S. W. direction. 

 The planes of stratification are inclined at an angle of from 10° to 15° 

 dipping towards E. 10° N. ; they are easily distinguishable here from 

 the smooth surfaces of cleavage by their dimpled and rippled super- 

 ficies. The cleavage planes are also marked by dendritic ^delineations. 



This ridge has been penetrated by a large dyke of basaltic green- 

 stone, running nearly E. and W., and branching in a N. and S. 

 direction. It is seen outcropping along the whole extent of the S. W. 

 base. At the N. E. base both branches disappear in the plain. The 

 basalt is also seen bursting through the strata at the saddle-shaped 

 depression on the summit of the ridge, where it has both a globular and 

 prismatic structure, the prisms pass into the globular form by the ex- 

 foliation of their angles, and I have even observed small spheroidal 

 nuclei in the exfoliated coats, which are in turn subjected to concen- 

 tric exfoliation. The dyke, like all others in this formation, does not 

 overspread or cap the rocks on its sides, but ends abruptly at the sur- 

 face. Towards the centre, like most volcanic dykes, it becomes crystal- 

 line and porphyritic, imbedding crystals of both whitish and pale 

 green felspar with a few of hypersthene and foliated hornblende. Aci- 

 cular augite is seen glistening in the more compact and quickest cool- 

 ed parts of the dyke, and occasionally cubes of iron pyrites. The ba- 

 salt melts easily into a greyish black glass. 



The shale in contact, both in the plain and on the saddle of the 

 ridge, is either hardened and rendered massive, compact or ferrugi- 

 nous, or is broken up, by crystalline forces apparently, into a number 

 of laminae often distinctly prismatic, and exhibiting dendritic marks 

 on the planes into which they readily split. At the base of the hill 

 the basalt and indurated shales assimilate so much at the] junction 

 line that it is difficult to distinguish them ; the shale has become dark 

 and hornblendic, and the basalt has acquired something of the fissile 

 structure of the shale. A similar phenomenon is observed in the me. 

 tamorphism of the hypogene rocks of Southern India, where the granite 

 near the point of contact acquires the structure of gneiss, and the gneiss 

 becomes in turn more granular, massive or granitoidal. The pheno- 

 mena presented by granite and basaltic greenstone at their contact 

 with metamorphic or other stratified rocks are extremely interesting ; 



