I. GEOLOGY. 821 



3252n. Geological Survey Map of Ireland, scale one 

 inch to one mile. 



The Geological Survey map of Ireland shows the state of progress to the 

 present time. The coloured maps are published, those uncoloured are in 

 various stages of progress, while the northern portions of Ireland have yet to 

 he geologically surveyed. 



The map shows the central plain of Ireland formed of Carboniferous Lime- 

 stone (coloured blue), having on the south the Galty, Commeragh, and other 

 mountains, formed of Lower Silurian and Old Red Sandstone, rising to eleva- 

 tions of over 3,000 feet. Along the east are the granitic and Silurian 

 mountains of Wicklow, reaching in Lugnaquilla a height of 3,039 feet. On 

 the west are " the Western Highlands " of Mayo and Galway, consisting of 

 granite, quartzite, schist, and other metamorphic rocks of Lower Silurian 

 age, in the midst of which are Upper Silurian beds on both sides of Killary 

 Bay. To the extreme south-west are the mountains and promontories of 

 Kerry, formed of Old Red Sandstone, with some Upper Silurian beds at 

 Sybil Head and Smerwick Bay. Carrantuohill, near Killarney, reaches an 

 elevation of 3,404 feet, being the highest point in Ireland. Small coal basins 

 (tinted dark), resting on flagstones and shales representing the Millstone Grit 

 series, occur in counties Cork, Tipperary, Kilkenny, Leitrim, and Tyrone. 



Edward Hull, F.R.S., Director. 



3252o. Drawings illustrative of the Microscopic 

 Structure of various English Eruptive Bocks, by Frank 

 Ilutley. Geological Survey of England and Wales* 



1. Micaceous Felstone, x 37. Potter Fell, LongSteddale, Westmoreland. 

 This rock is one of the numerous dykes which occur in the Upper Silurian 



rocks of the lake district, and may be regarded as a good typical example of 

 very many of them. It consists of crystals of Orthoclase and magnesian 

 mica embedded in a felsitic matrix. A little Magnetite is also present. Of 

 the different component minerals, only a few of the Felspar crystals are suf- 

 ficiently large to be apparent to the naked eye. The mineral components of 

 these dykes are the same as those of Minette, but the former rocks differ 

 from the latter in containing less magnesian mica, and in some of the Ortho- 

 clase oeing well crystallized. 

 la. Key to Fig. 1. 



2. Devitrified Pitchstone Porphyry, x 25. Stoke Lane, Mendips, Somer- 

 setshire. In this section crystals of both orthoclastic and plagioclastic felspars 

 are shown, containing some Viridite, and surrounded by the devitrified magma 

 which characterises this rock. 



2a. Chromo-lithographofFig. 2, by Messrs. Vincent Brooks, Day, and Son. 



3. Dolerite, x 75. Charfield Green, 150 yards north of Railway Station, 

 Gloucestershire. 



The larger drawing shows a much altered crystal of Orthoclase, as seen by 

 polarized light, exhibiting twinning upon the Carlsbad type. Several of the 

 larger crystals which occur in this rock, but which are not shown in the draw- 

 ing, may also be Orthoclase, while some of the smaller ones appear to be 

 Plagioclase greatly altered. Pseudomorphs after Olivine are also plentiful in 

 the rock, and the smaller drawing represents one of them. 



3a. Chromo-lithograph of Fig. 3, by Messrs. Vincent Brooks, Day, and 

 Son. 



4. Basalt, x 50. Damery, Gloucestershire. 



Composed of Plagioclase, altered Augite, and serpentine pseudomorphs, 

 some possibly after Olivine and Magnetite. 



