[Footnote 132: They are contained in the third volume of the _Transactions of the Geological Society_.] [Footnote 133: "Report on the Geology of the County of Londonderry and parts of Tyrone and Fermanagh," _Mem. Geol. Survey_, 1843.] While such advances were being made in the knowledge of the structure of the volcanic rocks of the North of Ireland, the geologist had already appeared who was the first to attempt a systematic examination of the Western Islands, and whose published descriptions are still a chief source of information regarding the geology of this extensive region. Dr. Macculloch seems to have made his first explorations among the Hebrides some time previous to the year 1814, for in that year he published some remarks on specimens from that district transmitted to the Geological Society.[134] For several years in succession he devoted himself with great energy and enthusiasm to the self-imposed task of geologically examining and mapping in a generalized way all the islands that lie to the westward of Scotland, from the remote St. Kilda even as far as the Isle of Man. From time to time, notices of parts of his work were given in the _Transactions of the Geological Society_. But eventually in 1819 he embodied the whole in his _Description of the Western Islands of Scotland, including the Isle of Man_. [Footnote 134: _Trans. Geol. Soc._ vol. ii. 1814.] This great classic marks a notable epoch in British geology. Properly to estimate its value, we should try to realize what was the state of the science in this country at the time of its appearance. So laborious a collection of facts, and so courageous a resolution to avoid theorizing about them, gave to his volumes an altogether unique character. His descriptions were at once adopted as part of the familiar literature of geology. His sections and sketches were reproduced in endless treatises and text-books. Few single works of descriptive geology have ever done so much to advance the progress of the science in this country. With regard to the special subject of the present memoir, Macculloch showed that the basalts and other eruptive rocks of the Inner Hebrides pierce and overlie the Secondary strata of these islands, and must therefore be of younger date. But though he distinguished the three great series of "trap-rocks," "syenites" and "hypersthene-rocks" or "augite-rocks," and indicated approximately their respective areas, he did not attempt to unravel their relations to each other. Nor did he venture upon any speculations as to the probable conditions under which these rocks were produced. He claimed that those who might follow him would find a great deal which he had not described, but little that he had not examined. Subsequent observers have noted many important facts, of which, had he observed them, he would at once have seen the meaning, and which he certainly would not have passed over in silence. But as a first broad outline