Criteria concerning sample and analytical data and stratigraphic control have been used in a critical assessment of the suitability, for use in construction of a Phancrozoic time-scale, of radiometric data relevant to the Carboniferous, Permian and Triassic periods. Few of the age determinations available in 1982 satisfy these criteria and many of those used previously as a basis for time-scales for this part of the Phanerozoic are considred unacceptable by present standards. On the basis of this review, ages of 365 ± 5 Ma, 290 ± Ma and 250 © 5 Ma. respectively, are proposed for the beginning of the Carboniferous. Permain and Triassic periods, and 205 © 5 Ma for the end of the Triassic. Radiometric ages are related. where possible, to the principal chronostratigraphic divisions of the rock successions representing those periods.
The principal aim of the science of geochronology is to present the history of the Earth as a dated sequence of events of known duration. The only available matrix of evidence is that contained within the rocks of the Earth, supplemented to a minor extent by meteorites and by rocks retrieved from the Moon. The primary requirement of geochronology is the erection of a measured geological time scale. By a painstaking study of the mutual relationships of the rocks of the Earth's crust and their included trace of the evolution of life, geologists over the last 200 years have slowly constructed a more-or-less detailed relative time scale. Precise physical measurements from which the ages of specific rocks and minerals can be estimated only became possible with the discovery of radioactive decay. The first estimates based upon physical measurements were undertaken in 1907. Great advances have been made in the last 15 years and the present state of geochronometry is summarized in this paper. There is a brief discussion of the assumptions, accuracy, precision and geological ' errors inherent in physical geochronometry. The five major physical methods of rock and mineral age determination currently in use are described. The evidence from which an estimate of 4600 x lo6 yr for the age of the Earth has been derived is reviewed and the history of the Earth is divided into a number of 'major cycles' (early Precambrian, 4600-3500 x lo6 yr; mid Precambrian, 3500-2400 x 106 yr; late Precambrian, 2400-1550 x lo6 yr and 1550-850 x 106 yr; Varisco-Caledonian, 850-250 x lo6 yr; and Alpine, 250 x lo6 yr to sometime in the future). Correlation of Precambrian rocks and the Precambrian time scale is almost entirely dependent upon physical age measurements. Our knowledge of the geochronology of the last 600 x lo6 yr of the Earth's history is based upon a careful combination of the evidence from biostratigraphy and physical geochronometry. This latter, or Phanerozoic, portion of the geological time scale is discussed in detail. I t is concluded that geochronology as a science is still in its infancy. A vast amount of age measurement work will have to be done before its principal objectives can be achieved.
The results of the 40 Ar/ 39 Ar step heating study of a folded and cleaved metasiltstone from Malmanare Point, Gerrans Bay, S Cornwall are presented. The rock comes from the mélange of the Roseland area, NE of the Lizard. The age spectrum obtained has two distinct sectors: (i) an older age component giving apparent ages of 521–490 Ma and (ii) a younger component with an apparent age of 424–410 Ma. The petrography suggests that the former is derived from the more resistant parts of large detrital muscovite flakes, while the latter comes from smaller metamorphic micas aligned along the prominent cleavage and also, in part, from original detrital grains partially or totally overprinted during the metamorphic event. The sediment was probably mid- to late-Ordovician in age. The source rocks appear to have been very largely of late Cambrian/early Arenig age. Folding, cleavage and low-grade regional metamorphism occurred as a result of late Caledonian orogenesis between 424–410 Ma. There is no evidence of any Variscan overprint on the age spectrum. Very minor, low energy argon losses appear to have taken place in mid-Mesozoic times (possibly related to regional mineralization) but, apart from this, the absence of any major overprint younger than 410 Ma indicates that since the end of the Silurian the rock has suffered only high-level, cold, non-penetrative deformation.
The Loire region of France (Fig. 1) is famous for its many chateaux which represent not only a pinnacle in art and architecture but also are a testimony to the excellence of the local building stone. The Saumur region of the Loire valley has an estimated 1000 km of quarried limestone galleries which were excavated from the 1lth to the 19th century. The stone has been used in buildings ranging from grand chateaux to simple domestic, commercial and agricultural buildings (Bailey 1993) (Fig. 2). The stone is extracted from thickly bedded, fine-grained Cretaceous limestones of Turonian age, mainly the Tuffeux Jaune of the Upper Turonian or the Craie Micacée of the Middle Turonian (Alcaydé et al. 1976). It owes its wide usage to the fact that it has ideal engineering properties for a dimension stone since it is stronger and more resistant to weathering than ordinary Cretaceous Chalk but not so strong as to be difficult to quarry or carve into decorative features. The stone occurs in deposits with widely to very widely spaced bedding and joint discontinuities which enable it to have been mined in a manner similar to that of the Jurassic, Bath Stone of England.The value of the stone does not lie solely in its architectural or constructional attributes. The abandoned mines remain as very stable underground spaces by virtue of their exceptionally wide bedding and joint spacing. They are relatively dry and, in common with most underground spaces, possess a constant temperature. Thus, they have found
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