Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/200

 has stripped away the basaltic masses which once covered them. This opinion seems principally grounded on the resemblance between these porphyries and those occurring in more ancient countries.

Another opinion represents these rocks as subordinate members in the flœtz trap formation. In favor of this it is urged that the general dip of the strata constituting the great basin of the basaltic area must (unless we suppose them to have been affected by a great dislocation and elevation) have carried the substrata on which the trap reposes to a much lower level than that which is actually occupied by the hillocks in question; and it has been asserted as a corroborating fact that the porphyry is actually seen to rest upon the basalt in one of the ravines which traverses the district, while still lower at Templepatrick, the stratum of chalk on which the basalt really reposes makes its appearance.

The occurrence of similar rocks subordinate to flœtz trap is by no means unexampled in other countries. Near Newry a narrow dyke of pitchstone porphyry extends for half a mile to the west, and in a continuation of the same line clay porphyry occurs. This dyke traverses sienitic rocks: an account of it is given in Dr. Fitton's notes on the mineralogy of the vicinity of Dublin, p. 53.

A singular formation of clay porphyry approaching to clinkstone porphyry occurs near Newtonglens in Antrim, seemingly associated with the old red sandstone; it is particularly described in the notes on the sections of the coast, appended to the present paper.