84' GEOLOCIY OF TRICIIIXOPOLY, &C. [CliAI'. IV. 1. 



cast and west of this contortion^ alternate beds of gneiss and ciystalline 



limestone stretch away in perfectly straight lines. In Loth these cases, 



intrusions of igneous rocks have taken j)lace to a greater or less extent. 



Two principal systems of jointing have been observed^ viz., one 



running north-south, (varying 5° or 10° to the east 



Jointing. 



or west,) the other running east-north-east, west- 

 south-west. There are also occasional cases of a system having a north- 

 west, south-east direction. The north and south line of jointing is most 

 constant and distinct, that running east-north-east — west-south-west, 

 being often coincident with the foliation. In the latter case, the con- 

 stancy of the dip (about 70° southwards) of the planes of jointing was 

 quite sufficient to distinguish it from the foliation. These various systems 

 are very well displayed round the base of the PatchamuUays, and the 

 faces of many of the finest precipices of the other mountain groups coin- 

 cide with some of the chief planes of jointing, of which examples will 

 be found further on. 



In some of the regions of trappean intrusion, the fissures of jointing 

 Joint fissure chaimels ^^ave been the channels along which the dykes 

 for trap. have been formed, as in the set of dykes in the 



neighbourhood of Volcondahpooram, and again in the trap -traversed 

 region south of Ahtoor, where the north-south system appears to have 

 been the line of least resistance, only three dykes in the former locality 

 being parallel to the east-north-east — west-south-west line. 



There can be no doubt that the outlines of the mountains depend 



Form of hiUs caused gi'^atly upon the systems of joints by which their 



^y joints. component rocks are divided. Many examples 



might be found, but we will quote only three, two of which strongly 



confirm this su^jposition — 



1st. — The tremendous precipices of Shenguttapand}^, on the north- 

 ern face of the Shairoar hill, the most northerly extremity of the green 

 hill ridges (on the Shevaroys), have their scarps coinciding with a plane 

 of jointing which lies north-west by 5' — 6" west, with a dip of 70" north. 



(306) 



