Prof, Hull — Structure of Porphyry. 449 



agreed with me, when I exhibited the fossil, that it was properly- 

 placed in the Lepidoptera (BrassoUnce, not " Satyridce"), I may 

 safely remain steadfast in my original opinion, reminding my friend 

 Scudder that though I am not infallible, neither is he, but at the 

 same time two heads are better than one. 



m. — On the Microscopic Stkuctube of the Lambat Pobphtey 



(ok Porphyrite).^ 



By Professor Edward Hull, M.A., F.R.S., President R. G. S. I. 



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 Eev. Dr. Haughton that he has iden- 

 tified 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 Eoyal College of Science in St. Stephen's Green; where, 

 to those familiar with the marbles of Italy and Greece, the appear- 

 ance of these specimens cannot fail to recall the green porphyry 

 [Marmor Lacedcemonium 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, how- 

 ever, though resembling each other in aj)pearance, 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 Eed Sandstone,^ as 

 this latter formation overlies the porphyry without appearing to have 

 undergone any alteration along the surface of contact.* 



The appearance of the porphyry is locally variable ; but in its 

 normal condition it consists of a dark green base, inclosing numerous 

 pale green crystals of felspar. These crystals are of various sizes up 

 to an inch in length. Occasionally there are cavities inclosing seg- 

 gregations of chalcedony, ejDidote, chlorite, or calcite, which also fill 

 in little veins and fissures in the rock itself In some places it is 

 highly vesicular, the cells containing flesh-coloured calcite. The late 

 Professor Jukes and Mr. Du Noyer appear to have considered the 

 rock to be hornblendic — attributing the dark colour of its base to the 



1 Eead before the Eoyal Geological Society of Ireland, 13th April, 1874. 



^ E. Hull, " Building and Ornamental Stones of Great Britain and Foreign 

 Countries," p. 73. 



2 Maps of the Geological Survey, Sheet 102, with Explanation by Messrs. Jukes 

 and Du Noyer. 



* It is represented in a similar manner in Griffith's Geological Map of Ireland. 

 Edit. 1855. 



DECADE II. VOL. I. NO. X. 29 



