220 PROCEEDINGS OF SOCIETIES. 



Dr. Griffith read the following — 



EXPLANATION OF THE PRINCIPLE OF COLOURING, AS WELL AS OF REFERENCE 

 TO THE COMPOSITION OF ROCKS BY LETTERS CONTAINED IN THE TABLE 

 APPENDED TO THE LAST EDITION OF THE " GEOLOGICAL MAP OF IRELAND;" 

 TOGETHER WITH THE ORIGINAL NOTES MADE IN THE YEAR 1847, RELA- 

 TIVE TO THE COMPOSITION AND STRUCTURE OF ARKLOW ROCK AND OTHER 

 IGNEOUS PROTRUSIONS OF THE COUNTIES OF WICKLOW AND WEXFORD. 

 BY RICHARD GRIFFITH, LL. D., F. G. S. LONDON AND DUBLIN. 



In preparing the last edition of my " Geological Map of Ireland," dated 

 April, 1855, I appended a Table explanatory of the colours and letters 

 adopted by me to indicate the character and composition of the several 

 rocks which occur throughout the country, as far as my examination 

 enabled me to do so ; but as it would appear from several sheets of the 

 Government Geological Survey of Ireland, lately published, on the scale 

 of one inch to a mile, that differences occur in the views entertained 

 by my friend Mr. Jukes and myself, in regard to certain portions of 

 the numerous insulated patches of rock of an igneous character, which 

 occur in certain continuous lines or chains throughout the lower Silu- 

 rian schistose district of the counties of Wicklow, Wexford, and Water- 

 ford, I think it desirable at the present time to explain to the Society 

 what is intended to be represented by the shades of red and purple 

 which distinguish those patches from the general tint of gray, which 

 indicates the position of the strata belonging to the lower Silurian 

 period. 



The different districts and insulated patches throughout Ireland 

 which are tinted with carmine indicate the areas which, in my opinion, 

 should be classed as granite ; and the difference in the composition and 

 ages of those rocks in different localities are shown by the letter IT, and 

 its adjuncts, as ITa, Ub, &c. The igneous protrusions, whether in 

 the form of mountain masses, hummocks, bosses, or dykes, composed 

 essentially of the minerals felspar and hornblende (the latter usually 

 predominating), generally known by the name of greenstone and green- 

 stone porphyry, are represented by a shade of deep-red and their varieties, 

 by the letter X and its adjuncts, as Xa, Xb, Xf, &c. ; while the several 

 protruded masses composed chiefly of felspar, with occasional horn- 

 blende and quartz, varying from sub-crystalline to compact (for one 

 variety of the latter of which (Xd) I have adopted Kirwan's term 

 'felsite' in my map), and which in Wicklow, Wexford, &c, are fre- 

 quently associated with protruded greenstones and greenstone porphyries, 

 are distinguished from them by the letters, though not by colour ; but 

 it should be mentioned that, partly owing to the smallness of the scale, 

 four miles to one inch, on which my Geological Map has been published, 

 and partly from want of sufficient leisure to make in detail all the ne- 

 cessary observations, I have been unable to distinguish on the Map nu- 

 merous comparatively small ramifications, whether of the compact 

 felspathic rock or of greenstone, which occur in the rocks which adjoin 



