of the North of Ireland, 



229 



assume a texture decidedly porphyritie. Among these nodules I 

 have found the following minerals. 





Minerals found imbedded in Dykes. 



Locality. 



1 



2' 



4 



5 

 6 



7 

 8 

 9 



10 

 11 



12 



Augite in angular fragments . . . 



Olivine in disseminated grains . . 



Crystallized glassy felspar . . . 



Compact felspar in distinct rounded 

 concretions. 



Radiated zeolite 



Green soft steatite, in distinct con- 

 cretions. 



Iron pyrites. 



Calcareous spar. 



Carbonate of lime, mixed with the 

 trap 



Isle -of Islandowey. 



Glen ofAlt.a-dara in the valley of Dunlughy 



Glen of Alt-a-dara. 



N.W. side of Arragh, highest dyke but one. 



On the shore on the Antrim side of Belfast 

 Lough. 



Glassy quartz in distinct concretions. 

 Sulphate of bai^tes. 

 Plates of mica. 



The mean specific gravity of the trap rocks forming dykes may 

 be rated at about 2.86. That of the specimens from Alt-a-dara, 

 containing olivine, being 3.14 ; that of homogeneous trap from the 

 dykes of the Giant's Causeway being 2.99 ; and that of the wacke 

 from the shore at Carrick-fergus being 2.45. 



The dykes are found traversing both the primitive and the 

 secondary rocks, nor have I ascertained in which they are the most 

 frequent. There appears to be no regular connexion between the 

 substance of the dyke and the rock through which it passes. I have 

 however sometimes found lime in considerable quantity in the dykes 

 that traverse limestone. Glassy felspar I have only found in the 

 dykes of primitive rocks. 



In the table of the observations of the dykes some of the rocks 

 cut by them are enumerated j I have added however another table 



