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HARD WIC ICE'S SCIENCE- G OS SIP. 



facility, the presence of potash being conspicuous. 

 But it happens that all these features are common to 

 another species, called microcline, which the best 

 authorities have recently differentiated as crystallizing 

 in the triclinic system. For a considerable time 

 microcline was regarded as an orthose felspar, until 

 the angle over the edges of the basal plane, and the 

 section corresponding to the clinopinakoid was ascer- 

 tained, after careful measurement byDesCloiseauxand 

 others, to possess a value as high, even, as 90 30', 

 which of course would place it amongst the triclinic 

 varieties, though in chemical composition and other 

 respects very closely approaching the monoclinic. 

 The great distinguishing feature, however, of micro- 

 cline is the peculiar appearance of its structure on the 

 basal section as viewed by a strong lens. It would 

 seem as if the surface were composed of fibres crossing 

 each other at right-angles, almost exactly resembling 

 the warp and weft arrangement of some coarsely- 

 woven material. The felspar in the district above 

 mentioned partakes largely of this character ; in fact, 

 it would be difficult to find a piece whose basal 

 section would not present more or less of the appear- 

 ance described. There are at present on my table 

 half-a-dozen pieces, taken at random from different 

 spots, in which the lines are visible to the naked eye. 

 The variety described is generally pure white, and is 

 different from the cream-coloured kinds found in 

 other localities. It is also largely sprinkled with 

 well-defined crystals of essonite, mentioned in a 

 former paper. The existence of the triclinic felspar, 

 albite, has been satisfactorily determined in this for- 

 mation, though it has only been found in very small 

 crystals in certain cavities in the granite, in a par- 

 ticular spot near Dalkey ; in general it seems to be 

 in the pasty condition, but its chemical composition 

 has been ascertained with accuracy by Rev. Dr. 

 Haughton, who has given a good deal of study to 

 these rocks. 



The mica is also a most interesting feature of this 

 district, both white and black varieties occurring in 

 well-defined masses. In some portions of the granite 

 the black appears to predominate, and contributes 

 to make the rock appear coarse, whilst in others the 

 white or margarodite mica (according to the above- 

 mentioned authority) is equally conspicuous. A very 

 remarkable feature in this latter kind is frequently to 

 be observed, and in fact is regarded by mineralogists 

 as peculiar to the district, and that is the arrangement 

 of the crystalline plates or scales in a distinctly plumose 

 form. In a hand specimen picked up from a loose 

 heap, and principally consisting of a ground mass of 

 felspar with occasional lumps of quartz, the mica is 

 seen to radiate from particular directions, suggesting 

 the form of a Prince of Wales's plume. In another 

 portion of the granitic formation situated atGlancullen, 

 some distance further inland, the plates of white 

 mica are not only largely developed, but show dis- 

 tinct traces of a feathery construction on the faces of 



the lamellae, which may be due to twining. This 

 district, however, offers so many remarkable features 

 as to merit a separate paper. In the hand specimen 

 alluded to above, the arrangement of the mica gives 

 a beautiful silvery appearance to the stone, but it is 

 to be remarked that the usual crystalline form of the 

 mineral is scarcely definable. In the white mica of 

 this district, the potash is found to exist in a pro- 

 portion of nearly eleven per cent., and iron peroxide 

 to the extent of 4.5 per cent. The crystals being 

 biaxial, the optical angles vary in certain localities 

 from 53 8' to 76 15'. 



The mineral known as schorl, or black tourmaline, 

 cannot be said to be a prominent constituent of the 

 granite in this neighbourhood — at all events to the 

 extent seen in the Cornish granites — though it would 

 not be correct to call it rare. It is well known that 

 schorl is frequently found in granite in the vicinity 

 of a junction between that rock and others, and 

 there is reason for believing that this is the case with 

 the granite under consideration, as it has been found 

 in Dalkey and other places adjacent to the mica 

 schist which immediately adjoins it. In examining 

 the structure of this latter rock, I picked up one day 

 a mass of schist in which several layers of a black 

 crystalline matter were embedded. Believing it at 

 first to be hornblende, I paid no particular attention 

 to it ; but a certain definition in the crystalline mass 

 of a distinctly different character from that of horn- 

 blende, combined with the absence of cleavage- 

 planes, calling for a closer examination, a hand-lens 

 soon revealed it to be black tourmaline. Further, 

 the application of Turner's test with bisulphate of 

 potash and fluor spar, by means of the blowpipe, 

 showed traces of boric acid which is generally as- 

 sociated with black tourmaline. In passing some 

 time since by a boundary wall of granite from the 

 district, I observed one of the blocks, about fifteen 

 inches square, to consist of crystalline felspathic 

 paste sprinkled with a black shining mineral in 

 radiated masses, which on close inspection proved 

 to be schorl, and probably this would be found to 

 be no isolated occurrence. 



Perhaps one of the most interesting facts to be 

 noted in the Killiney district is the occurrence of 

 chiastolite slabs in the mica schist. The mineral 

 is found in stellate masses on cleavage planes of the 

 rock, and, being of a dark colour, these forms are 

 easily distinguished against the brownish gray schist. 

 Beyond these peculiarities there is little to distinguish 

 the character of chiastolite, as its chemical formula 

 is practically the same as that of andalusite, being a 

 hydrated silicate of alumina with trifling accidental 

 admixtures of ferric oxide, magnesia, &c, and varying 

 according to locality. Andalusite is known to occur 

 in some abundance in the same rock formation, 

 though at some distance off in the adjoining county 

 of Wicklow. Specimens, however, have been found 

 now and again in the Killiney district, but at rare 



