III.—On the Microscopic Structure of the Lambay Porphyey (or Porphyrite)

Extract

This very handsome rock forms almost the whole surface of the Island of Lambay and of the adjoining coast, a few miles to the north of Dublin Bay. It is also laid open in the railway cutting at Donabate Station; and has received considerable notice owing to its intrinsic beauty as an ornamental stone, and its proximity to the Irish metropolis. I am informed by Kev. Dr. Haughton that he has identified specimens from the tessellated pavement of St. Kevin's Church, Glendalough, of the seventh century, which were submitted to his in spection by Sir W. Wilde. A polished table, together with a large block presenting two sides cut and polished, are placed in the gallery of the Royal College of Science in St. Stephen's Green; where, to those familiar with the marbles of Italy and Greece, the appearance of these specimens cannot fail to recall the green porphyry (Marmor Lacedæmonium viride) so largely used in ancient decorative works of art, and often reset in modern ones, the source of which is in the neighbourhood of Sparta and Marathon. The two rocks, however, though resembling each other in appearance, are essentially distinct as regards mineral composition, M. Delesse having proved that the Grecian porphyry contains augite as an essential, and is apparently a porphyritic basalt, while the Irish porphyry is altogether destitute of this mineral. As regards geological age, the Lambay porphyry appears to have been intruded amongst the Lower Silurian rocks at a period antecedent to that of the Old Bed Sandstone, as this latter formation overlies the porphyry without appearing to have undergone any alteration along the surface of contact.