﻿596 ME. J. V. ELSDEN ON THE IGNEOUS ROCKS [Aug. I905, 



consists. I found its specific gravity to be 2-73. Below are given 

 for comparison some analyses of similar rocks from other areas. 



I. II. III. IV. 



Silica 55-38 56"50 52-00 77'29 



Alumina 18-34 18-14 18-06 1462 



Ferric oxide 1-13 342 2181 . p 



Ferrous oxide 5'86 2"86 5'14/ 



Lime 3"25 3-38 4-59 trace 



Magnesia 3"47 1-22 2"84 0-38 



Potash 0-22 1-60 468 0-16 



Soda 7-12 5-28 3-78 7"60 



Titanic acid 0"90 0"85 0'98 n.d. 



Phosphoric acid trace ... ... n.d. 



Carbonic acid 2'00 511 3"59 



Water (at 110° Centigr.) 0-48 1 ^ ^ ^ 



Water (ignition) 2 # 39 J 



Manganese-oxide ... 0"25 



100-54 9932 99-93 100-62 



I. = Intrusive rock, Abercastle, Author's analysis. 



II, = Lime-bostonite of Msena. W. C. Brogger, 'Die Eruptivgesteine des 



Kristianiagebietes, III : Das Gang-Gefolge des Laurdalits' Christiania,. 



1898, p. 207 ; also ' Basic Eruptive Bocks of Gran ' Quart. Journ. GeoL. 



Soc. vol. 1 (1894) p. 26. 



III.==Bostonite, Onston Ness, Orkney Is. J. S. Flett, Trans. Boy. Soc. Edin. 



vol. xxxix (1900) p. 873. 

 rV. = Keratophyre, Wicklow. F. H. Hatch, Geol. Mag. 1889, p. 72. 



In some respects this rock is strongly suggestive of a bostonite^ 

 Mr. Cowper Reed's description of the smaller intrusives of Eoilna- 

 neena Cove (County Waterford), 1 would apply almost perfectly to 

 the Abercastle rock. Mr. Cowper Eeed was kind enough to send 

 me a slide of the Waterford bostonite-like rock for comparison, and 

 I find the resemblance very close. At the same time, the chemical 

 analysis does not altogether corroborate this view, since typical 

 bostonite not only exhibits a more completely-holocrystalline 

 structure, but has also the proportions of potash and soda approxi- 

 mately equal. There is, however, a very close resemblance between 

 bostonites and certain keratophyres, and the chemical composition 

 of the Abercastle rock is so similar to that of a rock described by 

 Prof. Brogger as lime-bostonite from Msena, Gran, Christiania, 2 

 that I am inclined to refer the Abercastle rock to that type. The 

 accompanying diagram (fig. 2, p. 598) represents graphically this 

 comparison. It is interesting to note that the lime-bostonites of 

 Msena are associated with a porphyritic variety, called by Brogger 

 msenite-porphyrite : this is the oligoclase-porphyry of 



1 ' Igneous Bocks of the Waterford Coast ' Quart. Journ. Geol. Soc. vol. lvi 

 (1900) pp. 677, 683. 



2 ' Die Eruptivgesteine des Kristianiagebietes, III : Das Gang-Gefolge des 

 Laurdalits ' Christiania, 1898, p. 207 ; also ' Basic Eruptive Bocks of Gran ' 

 Quart. Journ. Geol. Soc. vol. 1 (1894) pp. 23 et segg. 



