604 REPORT— 1886. 



posed in Anglesey and in the west and the north of Carnarvonshire ; they form 

 the greater part of the Malvern Chain, and crop out at the Wrekin ; they occur 

 on the south coast, at the Lizard, and in the district ahout Start Point and Bolt 

 Head ; they rise above the sea at the Eddystone. It is probable that these last 

 are the relics of a great mass of crystalline rock, which may have extended over 

 the Channel Isles to Brittany ; also, that we may link with the massif of the Scotch 

 highlands the crystalline rocks of Western Ireland on the one hand, and of Scan- 

 dinavia on the other. Among the indubitably igneous rocks we have granite, or 

 rocks nearly allied to it, in Scotland, in the Lake district, in Leicestershire, and 

 in Devon and Cornwall. Felstones, old lavas, and tuft's of a more or less acid type 

 occur in Southern Scotland, to some amount also in the Highlands, in the Lake 

 district, and in various localities of rather limited extent in West-central England, 

 as well as in the south-west region just mentioned, while in Wales we have, in the 

 northern half, distinct evidence of three great epochs of volcanic outburst, viz., in 

 the Bala, m the Arenig, and anterior to the Cambrian ' grits and slates. In South 

 Wales there were great eruptions at the last-named epoch and inOrdovician times. 

 I have passed over sundry smaller outbreaks and all the more basic rocks as less 

 immediately connected with my present purpose. It is, I suppose, needless to 

 observe that a coarsely crystalline rock, whether igneous or of metamorphic origin, 

 must be considerably older than one in which its fragments occur. 



Cambrian and later Pre-Cambrian. — That the majority at least of the gneisses 

 and crystalline schists in Britain are much older than the Cambrian period will 

 now, I think, hardly be disputed by an}' who have studied the subject seriously 

 and without prejudice. There are, however, later than these, numerous deposits, 

 frequently of volcanic origin, whose relation to strata indubitably of Cambrian age 

 is still a matter of some dispute. Therefore, in order to avoid losing time over dis- 

 cussions as to the precise position of certain of these deposits, or the particular bed 

 which in some districts should be adopted as the base of the Cambi'ian, I will asso- 

 ciate for my present purpose all the strata which, if not Cambrian, are somewhat 

 older. The latter, however, exhibit only micro-mineralogical changes, and of their 

 origin, volcanic or clastic of some kind, there can be no reasonable doubt ; so that 

 the difference in age does not appear to be enormous ; that is to say, I include with 

 the Cambrian the Pebidian of some recent authors. 



The utility of microscopic research has nowhere been better exemplified than in 

 the case of the oldest rocks of St. David "s. Some authors have supposed that the 

 base of the Cambrian series in this district has been ' translated ' beyond recognition, 

 others that it has been thrust out of sight by the intrusion of granitic rock. But low 

 down in the series, beneath the earliest beds that have as yet furnished fossils to 

 British palaeontologists, there is a weli-marked and widespread conglomerate ; under- 

 lying this, with apparent unconformity, comes a series of beds very different in aspect, 

 chiefly volcanic, and beneath this a granitoid rock. The conglomerate, in places, 

 even without microscopic examination, proves the existence, though probably at 

 some distance, of more ancient rock, as it is full of pebbles of vein-quartz and 

 quartzite ; but in other parts it is crowded with pebbles closely resembling the fel- 

 stones in the underlying volcanic group, and in some parts becomes a regular arkose, 

 made up almost wholly of quartz and felspar, closely resembling those minerals in 

 the granitoid rock, of which also small rounded pebbles occasionally occur. One or 

 two fragments of a quartzose mica-schist, which is not known to occur in situ in 

 the district, have also been found. It is therefore e\'ident that not only is the 

 volcanic series somewhat, and the granitoid rock considerably, older than the con- 

 glomerate, but also that an important series of rocks, some of which were thoroughly 

 metamorphic, was exposed in the district when the conglomerate was formed. I 

 have very little doubt that a study of the finer-grained sedimentary Cambrian beds 

 overlying the conglomerate will corroborate, were it needed, the conclusion which 

 the latter justifies. Passing on to North Wales the coarser beds in the Harlech 



' I take the base of the Arenig as the commencement of the next formation, 

 the Ordovician, which thus represents one phase of the Lower Silurian in the 

 variable nomenclature of the Geological Survey. 



