In Part IV of this paper the theory that the earth has continuously cooled down from a former molten state was shown to lead to limitations of temperature and to a distribution of rock-types at moderate depths which, taken together, made it impossible for igneous activity to have taken place as we know it to have done. It was argued that this conclusion definitely proved the theory to be wrong. Now, instead of deducing the distribution of rock-types in depth from their radioactive contents, the method of attack will be reversed and an attempt will be made to determine the downward distribution independently, so that the radioactive effects may be deduced without reference to any limiting hypothesis of a steadily cooling earth.

The present paper is a continuation of that published in the Geological Magazine, vol LXII, NO. IV, April, 1925 PP.145–80.

(1) 510 plants are recorded from various localities in the coalfield, fossils having been collected from several new localities.

(2) Fossil-plants are noted for the first time from the upper beds of the “Millstone Grit” of this district.

(3) Two new species of Sigillaria are described and figured.

(4) The known flora of the coalfield has been increased. Of the plants recorded—

6 species have not been recorded previously from Britain.

62 species have not been recorded previously from the coalfield, and

290 species have not been recorded previously from various localities in the coalfield.

Many of the species which have been previously recorded are here referred for the first time to definite horizons.

(5) The known vertical distribution of 38 species is extended to the Staffordian Series.

It is proposed to offer a few remarks as to the general relations of the rocks described to their associates in the Channel Islands, and in the Armorican Massif generally.

The genus Cyclus contains a number of small Arthropoda which are found in the Carboniferous rocks of Europe and the United States. As a rule the fossils are small, less than an inch in diameter, and very often they are enclosed in ironstone nodules. Consequently they are generally overlooked and are rarely found in collections.

The fossils described in the following pages were collected by Mr. James L. Begg, The Elms, Mount Vernon, Glasgow, fromthe Poolvash Limestone, Balladoole, Isle of Man. The fauna of these limestones, according to Dr. Wheelton Hind, indicates an upper Dibunophyllum age. The most recent and most complete list of Carboniferous Limestone fossils from the south of the I.O.M. is that published by Mr. John Smith, and compiled from the Survey list, from Dr. Hinde's list, and from Cuming's list, as well as from Mr. Smith's own collection.

In a former paper, a short sketch of the geographical and geological features of Western Togoland was given. The main geological divisions, excluding recent deposits, may be tabulated thus:-

In any purely stratigraphical treatment of a tectonic problem one of the greatest of the initial difficulties is always that of deciding where to begin, and in the present case this difficulty is unusually acute, since the primary object of the investigation is an endeavour to trace back phenomena occurring in later ages to causes of long antecedent origin. In more precise language, the object aimed at is to show that certain structural features of the Mesozoic rocks of the southern Midlands of England can be attributed to the recurrence of activity along old-established fold-lines of very early date.

It has become increasingly evident during the past few years that there are many geologists working on the older Palaeozoic rocks in various parts of the British Isles, to whom a knowledge of the common assemblages of graptolites characteristic of our different British graptolite zones might be useful. In the following lists, therefore, an attempt has been made to put together the facts gleaned from the study of the Graptolite Shales of the British Isles in many widely separated localities. The lists do not claim to be in any way a complete representation of the entire fauna of any zone, but merely an enumeration of those graptolites which in the author's experience are most universally and abundantly represented, so that they are likely to be the forms met with most characteristically in an exposure of any particular horizon. Some co-mingling at the boundaries of the zones must naturally be expected, especially when dealing with a succession of purely shaly deposits, but as a rule even then the coming in of new forms in abundance should be taken as an index of the passage to a higher horizon. This fact is one upon which great emphasis should be laid, it is upon this coming in of new forms, usually indicative of a more advanced stage in evolution, that the basis of modern zonal stratigraphy is laid; the persistence of old forms tends to vary greatly in different localities with the richness of the previous fauna; when the fauna of the earlier zones was both rich and varied, there are naturally more survivors into the succeeding zones than when the fauna was a poor one.

Barbados, occupying as it does the position of the most easterly isolated and the furthest out in the Atlantic Ocean of all the West Indian Islands, deserves special attention from geologists. Its rocks consist essentially of three units each separated from the other by an important unconformity, namely in descending order the Coral Rock, the Oceanic Series, and the Scotland beds.

In 1900 Mr. R. Bullen Newton described the casts and impressions of some lamellibranch shells, in blocks of fossiliferous sandstone from Kuala Lipis, Pahang.1 On account of the prevalence of specimens which he assigned to the genus Myophoria, Mr. Newton called the rock “Myophorian Sandstone”. Amongst other species identified by Mr. Newton Chlamys valoniensis has a wide distribution, “and is characteristic of the Ehaetic or uppermost Trias. It is to this horizon, therefore, that the Myophorian Sandstone must be referred”.

Recent papers and discussions have called attention to the fauna of the Yoredale facies of the Lower Carboniferous. These beds are well exposed in the valleys of Wensleydale and Swaledale in North West Yorkshire and they are there, perhaps, the most fossiliferous. As this is the type district for the Yoredale series, a brief statement of the more important faunal horizons would not be out of place.

The chief features exhibited by the heavy mineral residues from the various formations will now be summarized, but it is difficult to show, by tabulating or otherwise, the differences or resemblances between residues, although such may be quite obvious to the investigator. This is because the differences consist of a sum total of variations in size, shape, colour, and frequency of the mineralscontained in the residues under examination, and it is scarcely possible to indicate such differences in writing, without frequent repetition of words, there by involving a loss in clarity of expression, or if tables be employed, without losing sight of other connected data.

Ten years ago I published two papers in this Magazine under the above title. They constituted a first attempt to attack the new problems suggested by the universal presence of the radioactive elements in rocks, the discovery of which by the present Lord Rayleigh made it immediately necessary to abandon Kelvin's classical theory of the cooling of the earth. In 1913 I had already pointed out the possibility that Kelvin's problem might profitably be reversed, and that instead of calculating the age of the earth from its thermal condition, the latter, that is to say the distribution of temperature in depth, might be deduced from the earth's age. Two years later the new problems were brought to a tentative solution on the general assumption that the earth had cooled down from a molten state, an assumption which, although traditional, was at that time less clearly justified than it is to-day. As a working hypothesis it was further assumed that the total amount of the radioactive elements in the earth was limited by the condition that the earth had in fact cooled down and was not, as seemed to-be a possibility, growing hotter. Taking the age of the earth as Holmesthe Rockies (60 kins.), have recently been considerably increased. For these reasons geologists have rejected the new thermal contraction theory as being by itself still inadequate. My own opinion was that the theory explained about a third or a quarter of all the folding and overthrusting that has demonstrably occurred during the earth's known history.

In the Geological Magazine for May appeared an article by Dr. R. H. Rastall entitled “On the Tectonics of the Southern Midlands”, which I feel it absolutely necessary to criticize and correct in so far as it deals with evidence derived from Northamptonshire and quotations from my own papers, the latter in parts verbally accurate and yet altogether misleading as used; and I trust Dr. Rastall will accept the following corrections and additions rather as a contribution than an adverse criticism merely to his theory of “the recurrence of activity along old-established fold-lines of an early date–5, and will be separately referred to.

Within the last 50 years the lower reaches of the Teifi from St. Dogmells to Cardigan Bar have undergone a complete change: the bed of the river formerly lay near the west side of the estuary as far as the Webley Arms, and this point in the river was then known as Pwll Cam (“the crooked pool”), as it was here that the river took a sharp bend describing an ogee curve by way of the spit to the east of the present sandbanks: this old river channel lay in clay, unlike the present shallow river bed on the east side of the estuary, and quite big vessels could float even at low tide in front of the Webley Arms, which is now a low stretch of mud banks covered at high tide. A wreck, which laid up over two or three tides, was the simple means of effecting this change in the river's course. Since then it has led to other comparatively rapid geological changes: the Manian-fâch stream, which has its source in the Pant-y-Groes plateau, used to flow almost direct into the Teifi, when in its old course; now, however, since the bed of the Teifi, on leaving the Battery Point, crosses over to the east side of the estuary, the whole of the estuary to the west has been silted up with sand and mud, and this has been accelerated by the formation of sand-banks right across the mouth of the bay: consequently the Manian-fâch stream, on emerging off the mainland, is now deflected to the S.E. and flows for about a mile approximately parallel to the main river, though in the opposite direction, before joining it nearby the Battery Point. The watershed of the roughly triangular strip of sand flats, seen at low tide, lies very close to the present river bed, so that practically all the water off this area at low tide drains first into the Manian-fâch, and thus describes nearly a complete circle twice daily. This instance is analogous to, but not quite homologous with, that of the Eiver Aide in Suffolk, the original mouth of which has been deflected more than 12 miles south by currents from the north.

The Lower Greensand forms a well-wooded range of hills in Kent and Surrey which is known in Kent as the Ragstone Range. Between Ashford and Leith Hill this range is broken by the consequent streams of the Stour, Medway, and Mole, and by the obsequent streams of the Shode, Oxted Brook, and that part of the Eden known as Gibb's Brook.

The composition of chalk has been shown to consist dominantly of three types of material, in the order of their importance, (1) amorphous, or extremely fine-grained material, (2) spheres, and (3) organic remains.

The first, the writer believes, represents chemically precipitated calcite and aragonite, the second a chemically formed oolite, and the last the normal organic constituents.

Recent studies have shown that the surface waters of the ocean in favourable localities are saturated with CaCO3 and that variations in amount of CO2 mean either solution or precipitation. It is believed that the physical conditions of the Cretaceous sea favoured precipitation. Bacteria may have been a contributing factor.

It is hoped that the above suggestion of the possible chemical origin of a part of the chalk (the larger part) will cause those who have the opportunity of study to apply it further, for until the great amount of extremely fine material is accounted for the origin of chalk is not settled. The writer regards the chemical origin with favour because it seeks to explain the origin of chalk in the light of present knowledge of the precipitation of calcium carbonate from sea water.