As a consequence of their greater freshness and wider extent, and largely also because of the way in which they have been exposed along many leagues of picturesque sea-cliffs in the North of Ireland and the West of Scotland, they attracted attention at an earlier time than the less obvious volcanic memorials of older ages. The gradual development of opinion regarding the nature and history of volcanic rocks is thus in no small measure bound up with the progress of observation and inference in regard to the Tertiary volcanic series. I shall therefore begin this narrative by offering a rapid sketch of the history of inquiry respecting the Tertiary volcanic areas of the British Isles.
The basaltic cliffs of Antrim and the Inner Hebrides had attracted the notice of passing travellers, and their striking scenery had become more or less familiar to the reading public, before any attention was paid to their remarkable geological structure and history. In particular, the wonders of the Giant's Causeway and the Antrim coast had already begun to draw pilgrims, even from distant countries, at a time when geology had not come into existence. The scientific tourist of those days who might care to look at rocks was, in most cases, a mineralogist, for whom their structural relations and origin were subjects that lay outside of the range of his knowledge or habits of thought. In the year 1772 Sir Joseph Banks, together with Solander and a party, visited Staffa and brought back the earliest account of the marvels of that isle as they appeared to the sober eyes of science. His narrative was communicated to Pennant, together with a number of drawings of the cliffs and of Fingal's Cave. These were inserted by that geographer in his Second Tour, published in 1774, and from their careful measurements of the basaltic pillars and their delineation of the basaltic structure, are of special interest in the history of volcanic geology.
An intelligent appreciation of some of the geological interest of the region is to be found in the writings of Whitehurst,[125] who gave a good account of the basalt-cliffs of Antrim, and regarded the basaltic rocks as the results of successive outflows of lava from some centre now submerged beneath the Atlantic. More important are the observations contained in two letters of Abraham Mills.[126] This writer had been struck with the dykes on the north coast of Ireland, and was led to examine also those in some of the nearer Scottish islands. He believed them to be of truly volcanic origin, and spoke of them as veins of lava. A few years later, Faujas St. Fond made his well-known pilgrimage to the Western Isles. Familiar with the volcanic rocks of Central France, he at once recognized the volcanic origin of the basalts of Mull, Staffa and the adjoining islands.[127] His account of the journey, published in Paris in 1797, may be taken as the beginning of the voluminous geological literature which has since gathered round the subject. Three years afterwards (1800) appeared Jameson's Outline of the Mineralogy of the Scottish Isles. Fresh from the teaching of Werner at Freiberg, the future distinguished Professor of Natural History in the Edinburgh University naturally saw everything in the peculiar Wernerian light. He gave the first detailed enumeration of some of the eruptive rocks of the Hebrides, but of course ridiculed the idea of their igneous origin. Having heard of a reported "crater of a volcano" near Portree, he ironically expressed a hope that "there may be still sufficient heat to revive the spirits of some forlorn fire-philosopher, as he wanders through this cold, bleak country."[128]
[125] Inquiry into the Original State and Formation of the Earth, 2nd edit. 1786.
[126] Philosophical Transactions for 1790.
[127] Voyage en Angleterre, en Écosse et aux Îles Hébrides. Paris, 1797.
[128] It will be shown in a later chapter that there is a remarkably perfect volcanic vent near Portree, but the supposed crater referred to by Jameson was probably some little corry among the sheets of basalt.
The advent of Jameson to Edinburgh gave a fresh impetus to the warfare of the Plutonists and Neptunists, for he brought to the ranks of the latter a mineralogical skill such as none of their Scottish opponents could boast. The igneous origin of basalt, which the Plutonists stoutly maintained, was as strongly denied by the other side. For some years one of the most telling arguments against the followers of Hutton was derived from the alleged occurrence of fossil shells in the basalt of the north coast of Ireland. Kirwan[129] quoted with evident satisfaction Richardson's observation of "shells in the basalts of Ballycastle," and Richardson[130] himself, though the true explanation, that the supposed basalt is only Lias shale altered by basalt, had been stated in 1802 by Playfair,[131] continued for ten years afterwards to reiterate his belief in the aqueous origin of basalt. Thus the Tertiary volcanic rocks furnished effective weapons to the combatants on both sides. The dispute regarding the black fossiliferous rocks of Portrush had the effect of drawing special attention to the geology of the North of Ireland. Among the more noted geologists who were led to examine them, particular reference must be made to Conybeare and Buckland, who, in the year 1813, studied the interesting coast-sections of Antrim. The report of their observations gives an excellent summary of the arguments for the truly igneous origin of basalt, and a statement of opinion in favour of the view that the bedded basalts are the products of submarine volcanoes. Berger also about the same time described in fuller detail the geology of the Antrim district, and showed the rocks of the basalt-plateau to be younger than the Chalk. He likewise made a study of the basalt-dykes of the North of Ireland, and was the first to point out their prevalent north-westerly direction. The memoirs of these geologists[132] may justly be regarded, to quote the words of Portlock, as "the first effectual step made in Irish geology." Portlock's own description is still the most complete summary of the geology of that interesting region.[133]
[129] Geological Essays, 1799, p. 252, footnote.
[130] Richardson lived on the Antrim coast, and had daily opportunities of examining the admirable rock-sections there exposed. It was he who found the shells in supposed basalt, and led the geologists of his day astray on this subject. He made a clever but irrelevant reply to Playfair's plain statement of facts (Trans. Roy. Irish Acad. vol. ix. 1803, p. 481). His elaborate attack on "the Volcanic Theory" will be found in Trans. Roy. Irish Acad. vol. x. (1806), pp. 35-107. Though lively enough as a specimen of controversial writing, it forms, when seriously considered, rather a melancholy chapter in geological literature.