the North- eastern Part of the County of Antrim. -73 



Porphi/rt/. 



There are several varieties of this rock. The most common is a clay por- 

 phyry, of red, giey, brown, reddish grey, and bluish red colours, irregularly 

 intermixed in every possible manner. It contains crystals of glassy felspar, 

 vitreous quartz, and calcareous spar, and small crystals of brown mica'. The 

 crystals often disappear, and the rock passes into a claystone, or into an 

 argillaceous sandstone, closely resembling- the base of the finer varieties of 

 the old red sandstone. 



Another variety is a clinkstone porphyry, of a bluish or greenish colour, 

 which seems to pass, in some instances, into a close-grained greenstone. 



The area occupied by the porphyry is an irregular triangle, extending from Cushendall on 

 the north to a little beyond the Knockans on the south-west : its eastern boundary being the sea 

 between the mouths of the Cushendall and Balleask rivers. Within these limits the porphyry 

 is continuous, and does not crop out in independent masses, as stated by Dr. Berger 2. 



The strata of old red sandstone which are interposed on its western bound- 

 ary between the mica slate and the porphyry, preserving their usually high 

 inclination, dip uniformly eastwards, so as to pass under the porphyry. The 

 summit of Court McMartin, a hill immediately adjoining Cushendall, is com- 

 posed of porphyry, but the old red sandstone appears in its lower part. 

 The isolated hill Teavaragh, behind Cushendall church, has also a base of old 

 red sandstone. Strata of clinkstone porphyry crop out in its middle, while 

 its summit is composed of a disintegrating green-stone. Further, the newer 

 conglomerate on the shore covers the porphyry. It would appear from these 

 facts, that the porphyry overlies the old red sandstone, through which it has 

 been most probably erupted, and that it is covered by the new conglomerate. 



Few^ junctions being visible, there is little opportunity of observing altera- 

 tions such as igneous rocks usually produce upon the sedimentary deposits; 

 two instances, however, occur. At the mouth of the Cushendall river the strata 

 of the newer conglomerate are tilted by the porphyry from their usually low 

 inclination to a nearly vertical position ; but, as the contact of the two rocks 

 is concealed, it cannot be determined whether any change of structure is in- 

 duced upon the conglomerate. At the base of Teavaragh the old red sand- 

 stone is considerably indurated by the contact of the clinkstone porphyry 

 which overlies it. 



' Masses of jasper also occur. 



2 See Dr. Berger 's Paper, Geol. Trans., 1st Series, vol. iii. p. 201. 

 ' The following paragraph was added by the author, in April, 1836. 

 VOL. V. SECOND SERIES. L 



