2iK) 



On the Cri/stallin^ S/ruciurc of the 



[Apun 



formation of vertical prisms in liorizontnl 1)im1s of lavn, for in both cnsos, 

 ihe divisions whiili give riso to the prismatic structure arc nt. riglit an- 

 gles to the cooling surfaces. "Now in the Ir.ipof Amboor, I was csptuially 

 interested by finding, that, independent of these fissures at right angles 

 to the cooling surfaces, there were also distinct planes of cleavage, or as 

 miners call them " joints," which w hen combined with the fissures, render 

 the trap divisible into tliese very prismatic forms, spoken of by Mr. Lyi H. 

 I found it impossible to break the trap otherwise than into regul ar, and 

 prismatic fragments; anil this regularity of internal structure charly 

 proves, that crystallizing forces must have been in operation throughout 

 the wliole, to produce such striking results. The cleavage planes inva- 

 riably exhibited an ochreous hue, as if a portion of the oxide of iron, 

 which, in a greater or less proportion, ever forms a constituent of horn- 

 blende, had been decomposed, and determined towards these planes, by 

 an action analogous to that, by which certain bodies arc detcrniiucd to- 

 wards the different poles of a galvanic batter}'. 



Additional interest is attached to the phenomena above described, 

 when we learn they are members of a class which has been largely re- 

 cognised throughout the natural world. The following remarks by Pro- 

 fessor Sedgwick, will prove their existence throughout largo districts of 

 England and Wales. In both of these countries, the slate formation is 

 of great extent, and speaking of certain parts of it, he Irad examined 

 with much care, Professor Sedgwick remarks—" Besides the planes of 

 cleavage, formerly spoken of, we often find in large slate quarries, one 

 or more sets of cross joints, which, when combined with cleavage, divide 

 the rock into rhomboidal solids. These solids are not capable of inde- 

 finite subdivision into similar solids, except in one direction, namely 

 that of true cleavage, and in this wa}', we may, even in hand specimens, 

 distinguish the true cleavage planes, from the joints." This, I found, 

 was a matter of great ease in the trap of Amboor, for, independantly of 

 the ochreous hue before mentioned, as characteristic of the cleavage 

 planes, the joints or fissures being free from this, it was vain to attempt 

 to fracture a mass, save in the direction of the sides of the prisms, while 

 when the effort was made in this manner, division was easily accom- 

 plished. Speaking of the joints, Professor Sedgwick continues — -'I hese 

 last are fissures, placed at definite distances from each other, the masses 

 of rock betw^een thera having, generally speaking, no tendency to cleave 

 in a direction parallel to them." This remark is also applicable to the 

 trap of Amboor, the direction of cleavage never being parallel, butinva' 

 riably perpendicular, to that of the fissures. These last may be attri- 

 buted to the mechanical action, produced on the rock, either during con- 



