S4 On the Advancement of Geological Science in India. [Ja«, 



and also trap, are found in the SRme way. The hollow of the valleys 

 between the ridges of granite are filled wiih irregular beds, of generally 

 friable rocks of pegmatite, porphyries, hornblende slates, quartz rock, 

 of v\hich the porphyritic is the pievailing structure. The surface of 

 these beds is singularly level, and is covered generally with soil formed 

 from the disintegrated, and decomposed rock. Micaceous granite is 

 very scarce, or occurs only as a variety of the hornblendic gneiss, as a 

 formation, is unknown, and occurs either embedded in the granite or as 

 filling the interstices between its blocks. What has been called gneiss, 

 is hornblendic granite with a concentric lamellar structure, which is a 

 prevailing structure in the granite formation. The formation «)f which 

 the ralicondah hill is a type, called sienite by Dr. Benza, is a common 

 variety of granite, it forms the hills of Amboor, of Con^^oondy, and the 

 hills between the Cauvery, and CoUegaul in Coimbatore. It is felspa- 

 thic, and yields easily to the decomposing power of the weather; it has 

 no cleavage, and contains generally much mica, in veins or patches, and 

 trap dykes prevail in association with it. It is characterized by its 

 appearance, which is that of a craggy granitic mass, covered nearly to its 

 summit by a conical heap of large grained reddish gravel. A sphe- 

 roidal or concentric lamellar structure is common in the masses of 

 globular trap, which causes them to peel in pieces like bits of a rusty 

 shell ; this structure is original, and is in no way affected by tbe action 

 of the weather. The hills and masses of black granite and trap occur 

 as portions of the granitic formation, and can in no way be considered 

 as accidental coneretional varieties. Masses of hornblende felspar por- 

 phyry occur embedded in granite, proving thereby their prior origin. 

 Argillaceous soils in Europe are generally attributed to diluvial or allu« 

 vial action, but in India they also occur as original primary formations, 

 connected by veins with the subjacent rocks, and embedding masses of 

 granite in situ, as shewn by veins of quartz intersecting both rock and 

 matrix. The formation of this kind, prevailing at Bangalore, which may 

 be Cidled the Bangalore clay, is an immense and extensive tract of an 

 indurated argillaceous sandstone, which something resembles (as I am 

 informed by Mr. Burr) the new red sandstone of England: it lays im- 

 mediately u};on the subjacent granite, into which it graduates. I have 

 omitted to mention, that basalt also occurs in hornblendic granite, not 

 as dykes, or veins, but as an integral component part of the rock, and 

 in what may be called " lenticular" veins in the middle of the blocks. 

 It will be easily seen how irreconcilable son.e of these observations 

 are with prevailing theories, and, it is to be ht ped, that we may, in 

 future, find theories laid (;n the shelf a little, and allowed to take care 

 of themselves, while observers will confine themselves to a minute, and 

 accurate e-xamijaytion, and description oi jiature. I cannot close thefie 



