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II. — The History of Volcanic Action during the Tertiary Period in the British Isles. 

 By Archibald Geikie, LL.D., F.R.S., Director General of the Geological 

 Survey of the United Kingdom. (Plates I., II.) 



(Read 21st May 1888.) 



CONTENTS. 



Introduction, 



I. The Basic Dykes, .... 



§ 1. Geographical Distribution, 



2. Two Types of Protrusion, 



3. Nature of Component Bocks, 



1. External Characters, 



2. Internal Characters, 



3. Chemical Characters, 



4. Petrographical Nomenclature, 



4. Hade, 



5. Breadth, .... 



6. Interruptions of Lateral Continuity 



7. Length, 



8. Persistence of Mineral Characters, 



9. Direction, .... 



10. Termination Upwards, . 



11. Known Vertical Extension, . 



12. Branches and Veins, 



13. Connection with Intrusive Sheets 



14. Intersection of Dykes — -Repeated Dykes 



in the same Line of Fissure, 



15. Contact-Metamorphism, 



16. Relation of Dykes to Geological Structure, 



17. Data for Estimating their Geological Age, 



18. Origin and History of the Dykes, 



II. The Volcanic Plateaux, . 



1. Petrography, 



2. Areas of the Plateaux and Succession 



of Rocks in them, 



1. Antrim, . 



2. Mull, 



3. Small Isles, 



4. Skye, . . . 

 § 3. Vents of Eruption, 



a. Filled with Dolerite, &c, 



b. Filled with Agglomerate, 

 4. Intrusive Sheets, 



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III. The Bosses and Sheets of Gabbro, 



§ 1. Petrography, 



2. Relations to the other Volcanic Rocks 



a. Skye, 



b. Rum, ..... 



c. Ardnamurchan, . 



d. Mull, 



3. Structure of the Gabbro Areas, . 



IV. The Acid Rocks, .... 



§ 1. Petrography, .... 



1. Pitchstone and Trachyte Series, 



2. Felsite, Quartz-Porphyry, Grano 



phyre, and Granite Series, 

 2. Types of Structure, 



1. Bosses, 



a. Mull, . 



b. Small Isles, 



c. Skye, 

 Relation of these Bosses to 



other Members of the Vol 

 canic Series, 



(1) Relation to older 



Eruptive Vents. 



(2) Relation to the 



Bedded Basalts of 

 the Plateaux, . 



(3) Relation to the Gab- 



bros, 



(4) Relation to the Basic 



Dykes and Veins, . 



d. St Kilda, .... 



e. Antrim, .... 



2. Sills or Sheets, . . . . 



3. Veins and Dykes, 



4. Superficial Lava Streams, . 



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V. Summary, 181 



Introduction. 



Among the problems for the study of which the remarkably varied geology of the 

 British Isles offers peculiar facilities, perhaps none ranks higher in importance or in 

 general interest than the history of volcanic action. Placed on the oceanic border of 

 an ancient continental area, the region of Britain has lain within the limits where 



VOL. XXXV, PART 1. D 



