The Rev. Prof. Blake — Base of the Sedimentary Series. 315 



When we come to the details of the development, new questions 

 arise. With regard to the upper part, my own description, if some- 

 what fuller, agrees essentially with that of Dr. Callaway ; but with 

 the lower part this is not so. Dr. Callaway laid down a scheme of 

 succession as follows, beginning at the base : — 1. Halleflinta ; 2. 

 Quartz schists; 3. Grey gneiss; 4. Dark schist; 5. Granitoidite ; 

 every one of which were regarded by him as altered stratified 

 deposits. This sequence is naturally objected to by Dr. Hicks as 

 inconsistent with his own identifications. According to my own 

 observations, the halleflinta is of no stratigraphical importance^ the 

 granitoidite is an intrusive granite, and under the term " dark 

 schists" have been included two distinct groups of rocks, the truly 

 stratified chloritic schists, and the finely foliated hornblende schists. 

 Eliminating these, the remaining sequence between quartz schists, 

 grey gneiss and chloritic schist, agrees very closely with my own 

 arrangement. Dr. Callaway, however, has now come under the 

 influence of the theory that foliation is produced by dynamical 

 metamorphism, and this leads him to say that his "time succession" 

 no longer holds. How far the above arrangement is withdrawn is 

 not, however, quite clear. The character of the granite and halle- 

 flinta has nothing to do with this theory, since neither are foliated. 

 In the case of the hornblende schist, the stratigraphical evidence 

 of its sporadic development and its intrusive character have shown 

 that it is a truly igneous rock which may once have been a dolerite ; 

 but the same evidence is entirely wanting in the case of the chloritic 

 schist and grey gneiss, some of which latter Dr. Callaway now 

 regards as an altered felsite. 



The third question relates to the possible correlation of parts of 

 this series with rocks elsewhere described. The intrusive granite 

 has been called Dimetian. It may or may not be of the same age 

 as the granite of St. Davids ; the only connection between them is 

 that both antedate the Ordovician, and both are granites. The 

 " Halleflinta " has been called Arvonian ; but the "Halleflinta" in 

 Anglesey has no resemblance and no analogous position to any rock 

 at St. Davids or in Pembrokeshire ; the only connection between any 

 two such is that both have been called by the same wrong name, 

 neither being typical halleflintas. All the rest of the older rocks of 

 the island were at one time called Pebidian by Dr. Hicks ; they cannot 

 all be, and nothing is more certain than that they are all distinct 

 from the Bangor beds of Prof. Hughes, which have also been called 

 Pebidian. Prof. Bonney, in his British Association address, clearly 

 distinguished the two, and stated that the Anglesey rocks were 

 older than those of Bangor, which latter he referred to the Pebidian. 

 I myself originally referred the volcanic portion of the Monian to 

 the Pebidian, and called it the " St. Davids group," — a correlation 

 dependent only on the facts that both are of volcanic origin, and 

 both in some sense Precambrian ; but the rashness of such a 

 correlation is already clear. At the present moment, therefore, none 

 of the Precambrian or Monian rocks of Anglesey have been 

 definitely identified with any rocks elsewhere. 



(To be concluded in our next Number.) 



