assume a texture decidedly porphyritic. Among these nodules I have found the following minerals.
| Minerals found imbedded in Dykes. | Locality. | |
| 1 | Augite in angular fragments | Isle of Islandowey |
| 2 | Olivine in disseminated grains | Glen of Alta-a-dara in the valley of Dunlughy |
| 3 | Crystallized glassy felspar | Glen of Alta-a-dara |
| 4 | Compact felspar in distinct rounded concretions. | |
| 5 | Radiated zeolite | N.W. ride of Arragh, highest dyke but one |
| 6 | Green soft steatite, in distinct concretions | |
| 7 | Iron pyrites | |
| 8 | Calcareous spar | |
| 9 | Carbonate of lime, mixed with the trap | On the shore on the Antrim side of Belfast Lough |
| 10 | Glassy quartz in distinct concretions | |
| 11 | Sulphate of barytes | |
| 12 | 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 enumeratsed; I have added however another table
