1836.] 



Granitic Formation of Southern India. 



327 



being placed in 64! 0 ; on the line K L, with the extent from 64! 0 to B, 

 or E, let a faint arc be struck where the station A is supposed to be, 

 and in the same way from 83° on the line H I, with the extent from 

 83° to E or D, let another faint arc be struck intersecting the former ; 

 the place of intersection will be the place of the station A. Should 

 more intersection be required to ensure accuracy, let the angles subtend- 

 ed by D C be 56i then from 56! on the perpendicular to the line C D, 

 with the extent from 56!°, to C D, the arc will also intersect the two 

 former in A ; and so on for more angles. 



All this may be done while sitting on horseback, and the drawing in 

 of the features round the station immediately proceeded with, without 

 delay. 



The method is as accurate as when a plane table is used, and will 

 even shew the error arising from the angles not being reduced to the 

 horizon, where the plane in which they are measured is inclined to the 

 horizon 4 or 5 degrees. It is quite sufficient to lay down the scale of 

 the tangents to 5° as the true place of the centre is easily guessed. 



The method also possesses the advantage of protraction not being re- 

 quired in the field, and the great errors and inconvenience arising from 

 their use is thus avoided. 



VII. — On the Gra7iitic Formation, and direction of the Primary Moun- 

 tain Chains, of Southern India. — By Captain James Allardyce, 

 23a? Regiment Madras Light Infantry. 



Few countries afford so wide a field for investigating the phenomena 

 of the granitic formation as Southern India : in the north of Europe* 

 secondary formations so generally cover. the primary, that the latter 

 are in a great measure shut out from observation : in addition to this, 

 the more interesting fossiliferous strata occupy a large share of scien- 

 tific enquiry, while the less understood igneous rocks, having no key to 

 their history, are comparatively neglected. It is for these reasons that 

 inquiry regarding the position and arrangement of the early igneous 

 rocks in this country, is an especially appropriate study for those who 

 can direct their attention to such subjects; and so far from being a 

 hopeless task, it promises under all its advantages to afford much new 

 information. 



The circumstance most remarkable in the Indian granite formation is 

 perhaps the great prevalence of that kind of rock called primitive 

 trap, greenstone, or hornblende rock;* it does not form as in other 



* The Palaveram rock is a good example of the primitive trap : being nearly allied to 

 sienitic granite, it appears to be sometimes distinguished by the same name— it has becu 



