Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/237

 assume a texture decidedly porphyritic. Among these nodules I have found the following minerals.

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