3, 4, 6, 9, 10, 11, 15, and 14.. $JJtmaiwsaf fljt @ieaXagiraI!J!h&j?& EXPLANATORYMIDtOIl SHEETS 3, 4, 5 (IN PART), 9, lo, 11 (IN PART), 15 AND 16 OF THE MAPS OF TliE GEOLOGICALSURVEY OF IRELAND, COMPRISING NORTH-WEXT AND CENTRAL DON EGAL. BY EDWARD HULL, LL.D.; G. H. KINAHAN, M.R.I.A.; JOSEPH NOLAN,M.R.I.A.; R. J.CRUISE,M.R.I.A.; F. W. EGAN, B.A. ; J. R. KILROE; W. F. MITCHELL ; AXD A. M‘HENRY, M.R.I.A. WITH PETROGRAPHICAL NOTES BY J. S. HYLAND, PaD. Published by Order of the Lords Commissicmers of Her Majesty’s Treasury. DUBLIN: PRINTED FOR HER MAJESTY’S STATIONERY OFFICE, BP ALEXANDER THOM & Co. (LIMITBD), ABBEY-STBB~. -_ And to be pnrchaaed,either directly or through any Bookseller, flrom HODGES. FIGGIS L Co., 104, G-TON-STBEET,DWLIN;O~ EYBE & SPOTTISWOODE, EAST HAFCDINMTXBET, FLEET-~JTBBXI,E.C.; or JOHN MENZIES & Co.,l2, HANOVEB-STBEET, EDINBUME. and 88 and 90, W~sr NILE-STREET, GIABQOW, 1891. Pi-ice Five Shillings. LIST OF ILLUSTRATIONS. Fig. l--View of Errigal from the North, Front&piece Page Fig. 2.-Muckish Mountain, from near Ards, . 7 Fig. 3.-Distant view of Errigal from the S. E.? . , 8 Fig. 4.-Glenveagh, looking S. W. along the line of the great Gweebarra fault, . 9 Fig. 5.-Section at eastern entrance to Barnesbeg Gap, , 19 Fig. 6.- Section in Cliff N. of L. Reelan, . .. 31 Fig. 7.-Granite intruding into mica-schist, by roadside, three- quarters of a mi!e N. of Losset, . 49 Fig. &--View of Mackoght Hill, looking south-west, showing inversion of quartzite series, . 54 Fig. 9.-Section through Doon Reek, near Kilmacrenan, . 59 Fig. IO.-Doon Rock (old volcanic neck) looking N.E., Lough Salt Mountains in the distance, . 60 ./ Fig. Il.-Summit of Knock-na-Gra, three miles N. of Rathmelton, 64 / Fig. 12.-Errigal Mountain and Poisoned Glen, Dunlewy, looking I E. from the Falcarragh Road, . 70 ) F,.ia 13.- Granite dyke penetrating quartzite, near coast at Binanea-Falcarragh, . , , . 75 Fig. 14. -Diagrammatic Sketch of quartzite at Binanea, tra versed by veins of granite and pegmatite, running with and also oblique to the bedding , . Fig. 15.-Sketch-plan of banded foliated granite invaded by granite devoid of foliation, W. of Doocha.rry, . 79 Fig. 16.-Intrusive sheet of felstone-porphyry in sea-cliff at Ardsmore, . 83 Fig. 17.- Cave, north end of Cruit, . 85 Fig, 18.-Dyke of diorite intruded aming schists ; road near Bedlam, . , 91 Fig. lg.--Cliff at Scraigs Mountain, Glenfin, , . 95 Fig, 20.-Cliffs of foliated granite penetrated by dykes of basalt: in the Poisoned Glen, Dunlewy, . 96 Fig. 21 .-Tory Island, from mainland at Magheraroarty, . 101 Fig. 22.--View of the northern cliffs, looking west from near the u Wishing chair,” . 103 Fig. 23.-Rocks W. of the Lighthouse where the “ Wasp” was wrecked, . 104 Pig 24.-The Anvil, Doon peninsula, Tbry Island, L . 106 CONTENTS CHAPTER I, Page PHYSICAL GEOGRAPHY, . , . 5 Mountains, s . , . 6 Valleys and lines of depression, . 10 Rivers, . 10 Lakes and Mountain Laughs, . 11 Lakes due to solution of limestone, . 14 SeaJoughs, , . , . 6 14 Islands-Tory, Aran, , , . 16 CHAPTER II. ROUX FORMATIONSAND DIVISIONS, , . 8 . 16 CHAPTER III. GENERAL GEOLOGICALSTRUCTURE AND LITERATURE, . 16 General Smxessivn of the Metamorphic series, . 18 (1) Glencolumbkille Schists, . 1s (2) Lough Gartan Dolomite Group, : . 19 (3) Quartzite group of Knockalla, Knock Salt, &c., . 20 (4) Millford Schists, ’ . 20 Pakeontological EvicEelncesas regards the Geological Age of the Donegal Rocks, . I . 21 Lower Old Red 8andstone, . 23 LIST OF ‘PAPERS PUBLISHEDON THE GEOLOGYOF DO&JAI,, . 23 CHAPTER IV. DETAILED DESCRIPTION,. 29 CHAPTER V. LOWER OLD RED CONGLOMERATE, . , . 66 CHAPTER VI. IGNEOUSROCES-GRANITE, . 67 CHAPTER VII. FELSTONE-PORPHYRY, . 80 iV CHAPTER VIII. PAGE DIORITJZAND EPI-DIORITE, . 87 CHAPTER IX. BASALT ( DOLERITE) DYKE~, . 1 . 95, CHAPTER X. TORY ISLAND- General Description, . , . 98 Detailed Geological Description, . 102 CHAPTER XI. POST-PLIOCENE OR DRIFT DEPOSITS, . 107 CHAPTER XII. RECENT AND POST-GLACIAL DEPOSITS, . ll@ CHAPTER XIII. PRINCIPAL FAULTS, . 113, CHAPTER XIV. MINERALS, . l - . 114 CHAPTER XV. l GLACLATION. TABLE OF ICE-STZJIB, . 121 APPENDIX. PETROQRAPHICALNOTES ON THE ROCKS, . 129 . CATALOWE OF MINERALS, . 163 THE GEOLOGCICALSURVEY OF THE UNITEDEINCfDOM Is CONDUCTEDUNDEBTHEPOWERSOFTHE ~TH & 9TH VICT., CHAP. 63.-31~~ JULY 1845. DIRECTOR-QENEBALOF THE GEOLOGICALSURVEY OF THEUNITEDKIN~DO~ t ARCHIBALD GEIKIE, LLD., P.R.S. Geologica Survey @foe and Museum of Practical Geology, Jwmyn-street, London. IRISH BRANCH. O&w, 14, Hume-street, Dublin. DIRECTOR: EDWARD HULL,xA.,LL.D., F.R.S., F.Q.S. DISTRICTSURVEYOR: 0, HENRY KINAHAN, Y.R.I.A., &kc. SENIOROEOLOOISTS : R. Cl.SYMES, W.A.,F.G.s.; 5. B. N. WILKINSON ; J. NOLAN, M&U. ASSISTANTGEOLOGISTS: R. J. CRUISE, M.R.I.A.; F. W. EGAN, B.A.; J. R. KlLROE; W. F. MITCHELL; ALEX, M‘HENRY, Y.R.I.A.; A. B. WYNNE, F.Q.S.; and J. 5. HYLAND, Ph. D. FOSSILCOLLECTOR : R. CLARK. The observations made in the course of the Geological Survey are entered, in the first instance, on the Maps of the Ordnance Townland Survey, which are on the scale of six inches t-o the mile. By means of marks, writing, and colours, the nature, extent, direction, and geological formation of all portions of rock visible at the surface are laid down on these maps, which are preserved as data maps and geological records in the offlce in Dublin. The results of the Survey are published by means of coloured copies of the one-inch map of the Ordnance Survey, accompanied by printed Explanations. Longitudinal sections, on the scale of six inches to the mile, and vertical sections of coal-pits, &c., on the scale of forty feet to the inch, are also published, and in pre- paration. Condensed memoirs on particular districts will also eventually appear. The heights mentioned in these Explanationsare all takenfrom the Ordnance Maps. AQENTS APPOINTED FORTHESALE OF MEMOIRfl,MAPS,SECTIONS,ANDOTHERPUBLICATIONS OFTHE GEOLOQTCALSURVEY OFTHE UNlTED KINGDOM: LONDON : Mr. STANFORD (Maps), 5.5,Charing-cross, S.W. ; Meam. EYRE & SP~TTISWOODE, East Harding-street, for England. EDINBURGH: Messrs. JOHNMENZIKS & Co., 12, Hanover-street, Edinburgh; and 88 & 90, West Nile-street, Glasgow, for Scotland. DUB&IN: Messrs. HODOES,Fmors, & Co., for Irelaud. R PREFATORY NOTE. THE metamorphic rocks of Donegal and the north-west of Ireland belong to the same complex series which stretches northward into the Highlancls of Scotland. They present the same difficult problems of structure, and leave, in like manner as the Scottish rocks, their true geological age still to be fixed. That they are vastly younger than the Archxan gneisses of western Mayo and southern Galway is quite certain. There can be as little doubt on the other hand, that they are much older than the Silurian strata which stretch along their southern border. They are essentially a sedimentary series, though now for the most part crystalline schists.. They include, however, numerous bands of epidiorite, hornblende-schist, and various green chloritic schists which represent intercalations of igneous origin-sometimes, perhaps, contemporaneous lava streams, sometimes volcanic dust and ash, but perhaps, most frcquent,ly, intrusive sheets or sills. Much difficulty has been encountered in attempting to determine the true order of succession of these rocks, for they have been greatly plicated, fractured and inverted. Experience gained in the Geological Survey of the Highlands has shown that the apparent order may be entirely deceptive. In the present Memoir it has been decided to describe the various rock-groups as they appear to succeed each other; but it is quite recognised that the order ultimately asJcertained may be considerably different. I have recently satisfied myself that a core of Archaean gneiss rises from under the schists in County Tyrone. The local base of these younger metamorphic rocks must consequently be sought on the south-eastern border of the area, while the highest portions of the series are to be looked for in western Donegal. The few and obscure traces of fossils yet discovered among these schists, quartzites, and limestones do not afford any help towards the settlement of the question of geological age. ARCH. GEIKIE, Director- General, 15~ OCTOBER, 1890. PREFACE. THE geological examination of the district included in tars Memoir WENcarried out during the period ranging from 1883 to 1887 inclusive, by the Surveyors whose names appear on the title-page. The district is the most mountainous in the North-west of Ireland, and nowhere is exceeded in interest from the variety of the rocks it contains, and the intricacy of their structure and mutual relations. Owing to these circumstances the detailed mapping has been a work of more than usual difficulty; but I trust that an examination of the maps themselves will suffice to convince the observer with what care and ability my colleagues have accomplished their task, The structure of the district will be still further elucidated upon the publication of the sheets of Horizontal Sections now in course of engraving, and which are dra,wn across the country in directions best calculated to show both the form of the ground and its geological structure. The petrographical notes given in the Appendix, which are the result of microscopic investigation, will afford fresh interest to the study of the rocks, which have been subject to examination by numerous workers in the field of petrology, including the la.te Sir R, Griffith, Professor Harkness, Rev. Dr. Haughton, Mr. R. H. Scott, and Dr. Sterry Hunt. The glacial phenomena with which the region abounds, and which were first recorded by the Rev. Maxwell Close, have been kept in view, so that the observed directions of the ice-markings have been recorded on the maps, and are enumerated in Chapter XV.