228 Dr. Berger on ibe. Dykes 



able, as is also that of the dyke in the qnarries of Scrabo hill ; but 

 I am not able to state it with precision. 



There is not that variety in the substance of the dykes, that 

 their numbers, their distance from one another, and the varied 

 nature of the rocks which they intersect would lead one to expect. 

 I have found them composed of the following rocks, which are 

 introduced in the order of their most frequent occurrence. Trap 

 and greenstone, with their associates Lydian stone, flinty slate, 

 greystone and wacke. 



I have seen but one dyke of clay porphyry, viz. at Farland point 

 in Donegal, and I conceive it to be altogether of a different class 

 from those dykes to which my principal attention has been given 

 in the present paper. 



A dyke is formed either of a number of diminutive pillars aggre- 

 gated together, or of square rhomboidal pieces piled one upon 

 another like blocks of masonry, the long axes of these figures in 

 either case lying transverse, and perpendicular to the walls of the 

 dyke. These regular figures are often much disintegrated and 

 rounded, and sometimes assume the coated form ; the two appear- 

 ances being often united in the same portion of rock. 



Dykes are not, like metallic veins, divided into regular layers of 

 different stony substances; nor do we find in them those drusy 

 cavities which sometimes occupy the middle of metallic veins. 



The more compact the trap, the more apt is it to assume the 

 polyhedral form, to be homogeneous, and to be free from the por- 

 phyritic texture. In the hard variety I never found imbedded any- 

 detached mineral concretions, except a few small specks of soft 

 green steatite. When less compact, it is often set more or less 

 thickly with heterogeneous nodules, but seldom so abundantly as to 



