590 Me. J. Beete Jukes and the Rev. Samuel Haughton on the 



A short mineralogical and chemical description of this granitic axis has been 

 published in the twelfth volume of the " Quarterly Journal of the Geological 

 Society of London." The account which is here given is illustrated by many 

 additional analyses, which have thrown new light on some of the questions 

 discussed in that description. 



1. General Description of the Granite. — The granite of the axis, considered 

 in its most complex varieties, is a quinary granite, containing the following 

 minerals: — 



1. Quartz (watery). 4. White mica (Margarodite). 



2. White orthoclase (distinct crystals). 5. Black mica (Lepidomelane). 



3. Felspar (paste). 



Previous to discussing any particular analyses of the granite, it is necessary 

 to say a few words respecting each of these constituent minerals. 



2. Quartz. — The quartz of the granitic axis is the usual watery transparent 

 quartz of all the Irish granites ; it has a mean specific gravity of 2"645, as 

 determined from many specimens taken from veins containing large fragments 

 of quartz, accompanied by orthoclase and white mica, from localities in the 

 counties of Dublin, Wicklow, and Carlo w. 



3. Orthoclase. — This felspar is invariably found in the veins containing 

 large crystals, which are common, traversing the granite in every quarry that 

 is opened, with very few exceptions. The orthoclase is accompanied in these 

 veins by quartz and white mica, and occasionally by schorl, beryl, spodum.ene, 

 apatite, fluorspar, and garnet ; the latter, when it occurs, being always in 

 small crystals ; it is found more commonly than the other occasional minerals, 

 which may be considered rare, except in particular localities. 



Besides occurring in the large crystallized veins, orthoclase may readily be 

 distinguished in facets and flattish surfaces, in the general felspathic paste of 

 the granite of most parts of the axis. The composition of the orthoclase felspar 

 of the axis has been studied with care by the Rev. Professor Galbraith, who 

 has published the results of his analyses in the Proceedings of the Royal Irish 

 Academy, vol. vi., p. 134. The following Table contains the results of Mr. 

 Galbraith's experiments, which prove that the orthoclase of the different parts 

 of the granite axis varies very little in its character: — 



