1839] 



Trap Dykes in the S'lenite of Amhoor, 



291 



traction, ^vhile the fiised mass originally ejected from the interior of the 

 earth in a fluid, or semifluid, was passing into a solid state, the tension 

 thus caused, producing more or less regular sets of cracks, or fissures. It 

 is impossible, however, to attribute to a like origin the phenomena of 

 cleavage planes, they being distinctive of crystalline forces, and afford- 

 ing proofs, that the ultimate particles of the rock have been subjected 

 to chemical action, whereby this peculiar arrangement has been effected. 



In instituting the following enquiry into the causes to which these 

 phenomena are due, the first step will be to collect as many analogous 

 ones as possible, and thus to form a series of classes, whose relations to 

 each other may be observed, and a clue thus obtained, by which the 

 common cause operating on all, may be at length discovered as the 

 leading member of this class, and the head under which the others are 

 to be ranged may be j^daced. 



(1) . The phenomena of dykes, observed under different circum- 

 stances, both in the volcanic rocks of the present, and in those 

 of past eras. By a fair and close induction from observed facts, 

 geologists universally agree, that these rocks were originally eject- 

 ed in a fluid, or viscous slate, and have gradually assumed their 

 present hard, and compact nature, as they parted with the temperature 

 they then possessed. 



(2) . The slates of Cumberland and Wales, described by Prof. Sedg- 

 wick, the analogy between the peculiarities of whose structure, and 

 those of the trap dykes, has already been noticed, belong to a class of 

 rocks, to which Mr. Lyell has applied the term " metamorphic; " such 

 rocks, this eminent geologist considers, to have a sedimentary origin, 

 being originally deposited from water, but subsequent lo their deposi- 

 tion, he conceives them to have been modified and acted on by internal 

 heat, AVhether that heat was suflRcient to reduce them to the fluid 

 state, so that polar forces might act with greater freedom, is, as will af- 

 terwards be proved, a question of little, or no importance, as bearing on 

 the present enquiry. It is sufficient for this, that heat should have 

 been in action at all, its intensity being of comparatively little conse- 

 quence. 



(3) . Intimately connected with the two preceding classes, are the 

 phenomena exhibited by altered rocks, or those in the immediate vicini- 

 ty of decidedly volcanic products. These rocks are sometimes in ac- 

 tual contact with the originally fused masses, as when they form the 

 walls of a dyke, and then at the planes of junction on both sides and to 

 some distance beyond them, phenomena analogous to the preceding are 



