The volcanic geology of St. David's possesses a special interest inasmuch as it embraces a tolerably full development of various features which characterize the volcanic groups of later Palæozoic systems. Though the rocks are chiefly tuffs, they include also sheets of lava, as well as sills, dykes and bosses. They show a remarkable range in chemical composition from quite basic to highly acid materials. They present the amplest proofs of having been erupted and spread out over the sea-bottom, and they likewise afford clear evidence of alternation with the ordinary non-volcanic sediment of the time to which they belong. In these respects they are particularly noteworthy, for they prove that in the earliest Palæozoic ages the essential features of volcanic action were already as well developed as in any subsequent epoch of geological history. The volcanic group of St. David's attains a visible thickness of about 1800 feet. Its upper part graduates upward into purple and green Lower Cambrian sandstones. The base of the group is not seen owing to the plicated structure of the district. Hence the total thickness of volcanic material cannot be determined, neither can we tell on what it rests, whether on a still lower sedimentary series or on some platform of pre-Cambrian rocks. The structure of the group, notwithstanding all that has been written about it, has never yet been adequately worked out. The unfortunate and barren controversy about supposed pre-Cambrian rocks at St. David's has tended to obscure the real importance of these rocks as the oldest well-preserved record of volcanic action in Britain. They deserve to be carefully surveyed on maps of a large scale, in the same detailed manner as has been so successfully applied to the elucidation of younger volcanic tracts. Until such detailed investigation is made, any account of them which is given can be little more than a general outline of the subject. The following description is the result of my examination of the ground in company with my colleague Mr. B. N. Peach, and afterwards with the late Mr. W. Topley.[86] A few additional observations, from the subsequent exploration of Professor Lloyd Morgan,[87] are incorporated in the narrative. [Footnote 86: _Quart. Journ. Geol. Soc._ vol. xxxix. (1883), p. 294 _et seq._ While the essential parts of the investigation are given in the following pages, I would refer the reader to this paper for details not transferred to the present volume.] [Footnote 87: _Op. cit._ vol. xlvi. (1890), p. 241.]