1839] 



Trap Dyhes in ihe Sienite of Amloor. 



297 



the sun's heat than others, and the state of division in which the par- 

 ticles are, being also variable. 



In the ditch of the fort at Bangalore, I found the general depth of the 

 prisms to be about 4 or 5 inches ; in one place however this w^as increas- 

 ed to as much as 8. Sometimes the lines bounding the figures are 

 slightly curved, but these are exceptions to the general rule. In spite of 

 the irregularity which modifying causes have produced on these forms, 

 it is impossible to conceive they are due to simple mechanical con- 

 traction alone, and I cannot convey the view I am induced to take of 

 their origin better, than in the language employed by Professor Sedgwick, 

 relative to the analogous phenomena of the Welsh and Cumbrian slates. 

 " We may safely affirm" says this eminent geologist " that no contrac- 

 tion of dimensions, no retreat of parts in passing from a fluid to a solid 

 state, can explain such phenomena as these. They appear to me only 

 resolvable, on the supposition, that crystalline or polar forces acted 

 on the whole mass simultaneously, in given directions, and with a 

 definite power." Similar appearances to the above are to be observed in 

 the hard sun-dried soil so abundant in the neighbourhood of the canton* 

 ment of Bangalore ; also traces are to be noticed in chunam which has 

 dried rapidly, on being exposed much to the heat of the sun, combined 

 with the occasional presence of moisture. There are certain peculiari- 

 ties connected with this class, which, however, must be deferred, till the 

 general enquiry is more advanced — and therefore 1 proceed to the consi» 

 deration of the last of our series. 



(7) This class wnll include certain phenomena which have been exhi* 

 bitedby ice during a thaw, and which are strikingly analogous to the pre- 

 ceding classes, thus extending the law of the connection of crystalline 

 form with variation of temperature, to an extent which scarcely could 

 have been anticipated. The phenomena referred to, are described by Col. 

 Jackson in the Journal of the Geographical Society : Mr„ Lyell, to whose 

 work I am indebted for the account of them, quotes it to show, that the 

 phenomena are analogous to those of certain igneous rocks, but he 

 offers no explanation, and indeed says this tendency to a jointed struc- 

 ture is by no means understood : but it appears, from recent observations, 

 that ice sometimes presents a similar arrangement of parts. Scoresby,* 

 indeed, had long ago, when speaking of the icebergs of Spitzbergen, stat- 

 ed "that they are full of rents, extending perpendicularly downwards, and 

 dividing them into innumerable columns." Colonel Jackson has lately 

 investigated the subject with more attention, and has found that the ice 



* Arctic Voyages. 



