Shales-with-‘beef’ was the name given to some 70 feet of Lias on the Dorset coast, lying above (53) Table Ledge and below (76 a) the Birchi-Tabular. The beds consist of paper-shales, marls, indurated bands, and limestone nodule-beds, with numerous, more or less impersistent, interbedded seams of fibrous calcite, called ‘Beef’ by the Officers of the Geological Survey. Descending to the beach at Charmouth, and there forming reefs on the foreshore, the Shales-with-Beef are the most accessible Lias of that place. Yet they are, perhaps, the least known of all the beds. This is doubtless because of the generally unsatisfactory condition of the fossils found in them, and their consequent worthlessness on the one hand to the native, who finds no sale for such fragmentary and friable remains as the fossils present; while, on the other hand, the geologist seldom finds specimens more than approximately identifiable, and generally obtains completely satisfactory examples from but three or four horizons. Sir Henry De la Beche described the Lias of this coast nearly a century ago; and, although he generally under-estimated their thickness, the subdivisions that he then made can be approximately correlated as follows, at any rate those five which lie above the Blue Lias limestones:— (5) Irregular bed of Limestone, with nodular concretions, frequently containing ammonites; 2 feet=Stellaris beds (87–89). (4) Slaty marls, with several thin beds of indurated marl; 67 feet = Black Marl series above Birchi-Tabular and below Stellaris beds (77–88). (3) Slaty marls containing small crystals of selenite; 32 feet Shales-with-Beef (54–76).
SummaryThe Upper Triassic sediments are assumed to have contained concentrated solutions, which caused, or contributed to, their dehydration and contraction. The contraction of a colloidal mass results in the production of tension planes, and unless compensated by settlement, rupture finally ensues. Relief of pressure by rupture may bring the sohtions into the labile state, when innumerable closely-spaced centres of crystallization will be set up. The crystals, fed by vertically diffusing solutions, grow vertically, so long as contraction and feeding continue. When, at length, contraction attains its limit, further growth is resisted, and the stresses set up, transmitted to the central parting, produce bending and other pressure effects in the veins.
(A) Laminated Shales Shales, which are minutely laminated at the outcrop, but appear massive when traced inwards, have been described in Part I (p. 53). The change is accompanied by progressive bleaching towards the weathered surface, and by the deposition of minute, platy crystals of selenite between the laminæ. This bleaching suggests that the development of lamination has been accompanied by a loss of certain constituents of the shales, such as carbon (or hydrocarbons), and also (as indicated by the appearance of gypsum of iron-sulphide. Thus the weathering of the shales has produced a diminution of volume, and the resulting contraction, by separating the individual laminæ, has revealed the minute internal structure of the shales. It may be noted that similar development of lamination may be seen, for example, in the Rhætic Black Shales at Beacon Hill, Newark. In bed 71 e , consisting of well-stratified blue shale, Dr. Lang has found thin biconvex discs of a mineral determined by Mr. W. Campbell Smith as barytes. These discs are irregularly distributed along bedding-surfaces, from which they are readily detached, leaving behind a perfect impression of their shape and markings. The general appearance of these barytes discs can be gathered from fig. 3. The largest specimen examined had a mean diameter of 15 mm., a maximum thickness of 2 mm., and the specific gravity = 4·58. Both the upper and lower surfaces of the discs are marked by radial furrows, very distinct at the thin edge, but faint and often absent in
In the Geological Magazine for 1918 there appeared three papers discussing the origin of tile system of cracks ill septarian nodules. Dr. A. Morley Davies opened the discussion in a paper where, after a summary of earlier literature, he again put forward an older idea. Without adducing evidence, he considered : (a)that interposition of particles between those of the original matrix caused expansion which set up tensile strain in the growing body ;(b)that the cracking of the nodule was due to the relief of those tensile stresses.
The St. Austell granite-mass has attracted considerable attention in the past, because of the economic importance of its mineral deposits and the petrological problems that arise out of the wonderful display of pneumatolytic activity manifested in the district. The principal references to the literature are given in the footnote, and since these contain full bibliographies, it is unnecessary to compile another for the purpose of this paper. Two reasons induced me to undertake this work. In the first place, it appeared desirable to develop and apply to a coarse-grained igneous rock-mass quantitative microscopic methods. Secondly, it seemed probable that much light could be thrown upon the problems of the St. Austell granite by quantitative and qualitative data obtained from a larger number of representative slides than has been available hitherto. A preliminary investigation in the field soon revealed that the district presented some disadvantages in the application of the method, owing to the occasional occurrence of very coarse porphyritic structure in the granite; but, at the same time, it was evident that the results would outweigh a little uncertainty arising from this difficulty. The present paper deals solely with the granite, and all post-consolidation and metamorphic changes are excluded from its scope. The aim has been to ascertain what evidence exists of differentiation and variation in the original rock. Certain pneumatolytic effects have been generally recognized as inextricably bound up with stages in the intrusion and crystallization of the original magma, and such, of course, come within the purview of
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