THE TERTIARY VOLCANOES 
BOOK VIII 
1 16 
task, I was able to complete a discussion of the whole history of Tertiary 
volcanic action in this country, which was communicated to the Royal 
Society of Edinburgh in the early summer of 1888. 1 Since that time I 
have continued the research, and have from time to time communicated 
my results to the Geological Society. These various memoirs are combined 
with hitherto unpublished details in the following account of the British 
Tertiary Volcanic Rocks. 
Professor Judd has also prosecuted the investigation of the petrography 
of the rocks, and has published his observations in the Quarterly Journal of 
the Geological Society? To these papers by him more detailed reference 
will be made in later Chapters. 
In describing the geological history of a great series of rocks, chrono- 
logical order is usually the most convenient method of treatment. Where, 
however, the rocks are of volcanic origin, and do not always precisely 
indicate their relative age, and where moreover the same kinds of rock may 
appear on widely-separated geological horizons, it is not always possible or 
desirable to adhere to the strict order of sequence. With this necessary 
latitude, I propose to follow the chronological succession from the older to 
the newer portions of the series. I shall treat first of the system of dykes, 
by which so large a part of Scotland and of the north of England and Ireland 
is traversed. Many of the dykes are undoubtedly among the youngest 
members of the volcanic series, and in no case has their age been as yet 
determined except relatively to the antiquity of the rocks which they 
traverse. They must, of course, be posterior to these rocks, and hence it 
would be quite logical to reserve them for discussion at the very end of the 
whole volcanic phenomena. My reason for taking them at the beginning 
will be apparent in the sequel. After the dykes, I shall describe the great 
volcanic plateaux which, in spite of vast denudation, still survive in exten- 
sive fragments in Antrim, the Inner Hebrides and the Faroe Islands. The 
eruptive bosses of basic rocks that have broken through the plateaux will 
next be discussed. An account will then be given of the protrusions of acid 
rocks which have disrupted these basic bosses. The last chapters will con- 
tain a sketch of the subsidences and dislocations which the basalt-plateaux 
have suffered, and of the denudation to which they have been subjected. 
As has been explained in Chapter iii., the volcanic cycle of any district, 
during a given geological period, embraces the whole range ol erupted pro- 
ducts from the beginning to the end of a complete series of eruptions. 
Reference was made in Book I. to the remarkable variation in the 
character of the lavas successively poured out from the same volcanic reser- 
voir during the continuance of a single cycle, and it was pointed out that 
Richthofen’s law generally holds good that while the first eruptions may be 
1 Trans. Roy. Soc. Min. vol. xxxv. part ii. (1888), pp. 23-184. 
2 Quart. Journ. Gaol. Soc. vols. xlv. (1889), xlvi. (1890), xlix. (1893). In tlic first of these 
volumes Professor Judd offered a detailed criticism of my views as to tlxe order of succession and 
history of the volcanic rocks of the Inner Hebrides. Subsequent investigation having entirely 
confirmed my main conclusions, it is not necessary to enter here upon matters ol controversy. 
Reference, however, will be made in subsequent Chapters to some of the points in dispute. 
