1 2 Observations on the Geology 



ture, of course, the water will dissolve a somewhat larger por- 

 tion of the salts, and the effect will be proportionably greater. 

 Thus the most powerful mixture, given in my table of frigo- 

 rific mixtures, consisting of phosphate of soda, nitrate of am- 

 monia, and diluted nitric acid, will, when mixed at the tem- 

 perature of 50°, produce a cold of 21° below 0; and if mixed 

 in due proportions at 100°, it will produce, in an instant, a 

 cold of 20°; viz. a reduction of eighty degrees. By means of 

 this mixture, as I have been informed, water has been frozen 

 solid " under the line." I am, Gentlemen, 



Your most obedient servant, 

 Oxford, June 10, 1828. RlCHARD WALKER. 



V. Observations on the Geology of the Hyderabad Conntiy*. 



r T , HE country around Hyderabad is composed entirely of 

 ■*■ granite, intersected by quartz, which generally runs north 

 and south ; and by trap, which has no definite direction. 



The hills are generally in ridges. In some instances they 

 are insulated, of a mamillary form, or abrupt and precipitous. 



The ridges are covered with detached masses of rock, and 

 frequently (when seen at a little distance) hare more the ap- 

 pearance of heaps of loose stones than of solid hills. The 

 mamillary hills are almost always devoid of vegetation, having 

 a smooth surface, with large detached lamellae lying loosely 

 on their sides, and apparently ready to slide or tumble down 

 oh the slightest impulse into the neighbouring valleys. 



The insulated hills often present on one or more sides a 

 smooth, perpendicular surface, which makes a very sudden 

 curve at the top, or undulates, and thus contracts the summit 

 of the hill. 



Sometimes we find the surface of the granite forming part 

 of an immense curve, and rising very gently and to a small 

 height above the surrounding plain. In other instances it is 

 waved, and presents a great variety of outline. 



Huge blocks of granite are every where strewed over the 

 country, and are often piled over each other in the valleys, or 

 on the sides or summits of the hills, giving rise to the most 

 fantastic shapes, and often closely resembling ruined buildings. 

 It is not uncommon to see three or four immense masses of 

 granite placed above each other, with their surfaces nicely 

 adapted, having somewhat the appearance of the ruin of an 



* From the Transactions of the Literary Society of Madras. Part i. page 79- 



ancient 



