October 20, 1921] 



NATURE 



257 



the "jubilee number." O. Bergstrand writ^ on the 

 iriTective wave-lengths of the galactic stars; A. S. 

 Eddington on the dynamical equilibrium of the stellar 

 system ; P. Guthnick and A. A. Nijland on the 

 Cepheid jM-oblem ; J. G. Hagen on dark nebulae; 



H. Shapley on the galactic distribution of the B stars ; 

 and H. v. Zeipel on the masses of stars in clusters. 

 The list is not exhaustive, but will serve to give an 

 idea of the varied contents of this memorable pmb- 

 lication. 



AS is pointed out by the Director in the preface 

 to the memoir under notice, the district, though 

 not containing many deposits of economic importance, 

 includes a very extensive series of rocks, ranging 

 from the Ordovician to the New Red, and the hope 

 is expressed that the district will be recognised as a 

 typical area for the study of the formations repre- 

 sented, as developed in S.W". Wales. 



The area shows considerable physical diversity. 

 The highest ground is formed by remnants of a 

 plateau mainly consisting of Lower Old Red Sand- 

 stone. The remnants are separated from one another 

 by erosion-valleys and by level tracts chiefly com- 

 j>osed of Carboniferous Limestone. The latter are 

 referred to as the limestone fiats, and considered to 

 be probably the work of the Pliocene sea. The re- 

 markabh- level character of these flats is shown in 

 the frontispiece. The coast is deeply indented by 

 partly drowned valleys (riasj, of which the chief is 

 Milford Haven. Manj- of the valleys are independent 

 of the geological structure, and afford examples of 

 superimposed drainage. 



The oldest rocks occurring in the district are shales 

 and sandstones belonging to the Llanvim series ex- 

 posed in the anticlines of Freshwater East and Castle- 

 martin Corse. Here, too, are seen Silurian rocks, 

 which both lithologically and in fossil-contents are of 

 the Welsh-borderland type. 



The Old Red Sandstone is specially interesting from 

 the intercalation in the upper beds of bands contain- 

 ing a marine Devonian fauna. These bands, which 

 were originally noted by de la Beche, and afterwards re- 

 ferred to by Salter, have yielded more than lift}- species 

 of fossils, by far the greater number coming from 

 Freshwater West. The author considers that apart 

 from these marine intercalations in the highest beds. 

 he Old Red Sandstone represents an aqueous deposit 

 ; rmed under "continental " conditions. He does not 

 believe that any of the rocks are directly aeolian, the 

 sandstones being too well bedded and conglomeratic 

 to represent sand-dunes, and though the marls may be 

 formed of wind-borne dust, it probably settled in water. 



The Old Red Limestones "probably represent pre- 

 <apitates thrown down as the fresh, tributan.- waters 

 carr\-ing the calcium carbonate in solution mingled 

 with the more saline water of the basin of deposi- 

 tion." The breccias and conglomerates of the Upper 

 Old Red contain much igneous material, chiefly acid 

 lavas, which. Dr. H. H. Thomas points out, "show a 

 general resemblance to the pre-Cambrian and lowest 

 Palaeozoic rocks of Pembrokeshire, more particOlarlv 

 of the part north of St. Bride's Bav." In the Ridge- 

 way conglomerate of the Lower Old Red Sandstone 

 the pebbles are chiefly quartzite, igneous material 

 being almost unrepresented. 



The Carboniferous Limestone Series (Avonian) is 

 perhaps the most interesting formation in the area, 

 and the full account now available will be most 

 acceptable to all students of these rocks. The author, 

 like all other workers on the Carboniferous Lime- 

 stone, has been deeply influenced by the work of the 

 late Dr. Vaughan, to whom he "cannot adequately 

 express his indebtedness." The faunal subdivisions 



■ Memoirs of the Geoloeical Survey : England and Wales. " The Geology 

 :; he South Wales Coalfield. "' Partis: "'The Countrj- around Pembroke 

 -■"'} Tenby." Bv E. E. L. Dixon. (Southampton :' Ordnance Survey 

 Cmce; London: K. Stanford. Ltd., 1921.) oj. net. 



Geology of the South Wales Coalfield. ^ 



recognised are in the main those of Vaughan 's 

 original paper, but the author draws the line between 

 the Upper and Lower Avonian at the top of the 

 C, beds, where a marked transgression occurs in the 

 northern part of the South Wales area, instead of at 

 the top of Cj, where Vaughan originally drew it. 



The studv of the rock-types of the Carboniferous 

 Limestone Series is one which the author has made 

 peculiarly his own, but he has already treated the 

 subject so fully in describing the rocks of Gower 

 that there is comparatively little of a novel char- 

 acter in the present memoir. .\ description is given 

 of the interesting reef dolomites, and of the char- 

 acters which lead the author to compare them with 

 the reef or knoll limestones of Clitheroe and the 

 Belgian Waulsortian. They occur in the C beds of 

 the extreme south-west comer of the district, and 

 appear to be essentially bnozoan reefs. Oolites have 

 been recognised at an e.\ceptionaUy large number of 

 levels in the Lower Avonian. In ver}- numerous re- 

 spects mentioned by the author the rock-t\'pes are 

 identical with those of the Bristol district. The term 

 ■' Zaphrentid-phase," which was introduced by 

 Vaughan, but not defined, is employed by the authcw, 

 who defines his use of the term. A ver}- lengthy 

 fossil list is included. 



The "Millstone Grit" — the term used to include 

 the sandstones and shales inter\-ening between the 

 Carboniferous Limestone and the Coal Measures — 

 though well exposed, is greatly disturbed, and the 

 strata are difficult to correlate. The lower beds are 

 shown to contain radiolarian chert and fossils of 

 Pendleside type, while the presence of certain plants 

 in the upper beds appears to indicate an horizon as 

 high as the Middle Coal Measures of the Midlands. 



Certain deposits of a peculiar character preserved 

 in fissures or cavities of the Carboniferous Limestone 

 are the only ones referred to the New Red (Trias). 

 The most remarkable of these are the gash-breccias, 

 which are fully described, and illustrated by an 

 admirable plate. The author considers that they are 

 probably due to the collapse of the roof and ^ides of 

 cavities formed by the underground solution of the 

 limestone. 



S. particularly full and interesting account is given 

 of the earth-movements which have affected the 

 district, and while by far the most important are the 

 post-Carboniferous (Armorican) movements, others 

 occurred between the Llanvim and Wenlock periods, 

 between the Ludlow and the Lower Old Red Sand- 

 stone, between the Upper and Lower Old Red Sand- 

 stone, between the Upper and Lower Avonian, and 

 between the Carboniferous Limestone and Millstone 

 Grit. .All the chief strike-faults are overthrusts of 

 Armorican date, while the cross-faults, which are 

 tabulated, also appear to belong in the main to the 

 same period of disturbance. 



The district differs from Gower and some other 

 parts of South Wales in that glacial deposits are 

 nowhere seen resting in clear sequence on undoubted 

 raised-beach. 



The memoir is illustrated by five fine plates and 

 an admirable series of sketch-maps. It every- 

 where bears evidence of the minute observation and 

 thoroughness which are so characteristic of the 

 author's work. 



NO. 2712, VOL. I08I 



