Summary The Ordovician inliers of the Cautley and Dent districts (Westmorland and NW. Yorkshire) expose an almost complete sequence of Ashgill beds—which form a better type sequence than that in Ash Gill, Coniston—underlain by the Pusgillian and part of the Onnian Stage of the Caradoc Series. Eight zones are recognized in the Ashgill Series, characterized by distinctive trilobite and brachiopod assemblages; faunal lists are appended. The term “Cautley Mud-stones” is introduced for strata from the Onnian Stage to the top of Zone 7. Marr’s “contemporaneous volcanic group” and “ mucronatus band of the Staurocephalus group” are renamed the “Cautley Volcanic Group” and “Cystoid Limestone” respectively. The term “Wilsey Beck Sandstone” is proposed for a rock unit that forms a useful marker horizon in Zone 2 of the Dent district. The approximate maximum thickness of Ordovician beds exposed is 2,100 feet (640 metres). Tentative correlations are made with Ashgill successions in Great Britain and abroad.

This paper is a revision of the standard chronostratigraphy of the Ordovician of the historical type area in England and Wales. The revision is a response to the need for more precise definitions of series and stages, especially for practical international correlation. In some cases this has entailed moving horizons for Series bases away from those in the classical sections. The scheme will be used as the standard for a forthcoming revision of the Correlation Chart of the British Ordovician System. The Ordovician is divided into five series: Tremadoc, Arenig, Llanvirn, Caradoc and Ashgill. The Llanvirn is extended to include part of the classical Llandeilo Series, which is included within it as a stage. Where stage subdivisions of standard series have not yet been defined, as in the Tremadoc, they are proposed in this paper. However, Bancroft’s fine stadial divisions of the Caradoc, which have been criticized for their brevity and local utility, are combined into four new stages, which will have wider application. The former Caradoc subdivisions are retained as substages for the purposes of regional correlation.

The 'White Rock', constituting a portion of what may be a lacustrine sedimentary sequence near the margin of a crater c. 90 km across in Sabaea Terra, Mars, measures 18 15 km 180-540 m high. It is re-interpreted as a lens of magnesium carbonate precipitated where ground waters seeped into an ancient evaporating crater lake. Were life to have emerged on Mars, as seems feasible, then the 'White Rock' might be expected to comprise a complex of stromatolitic mounds. Salda Gölü (Lake) in Turkey, is taken as an analogue. This enclosed lake is nearly surrounded and underlain by partially serpentinized harzburgite. Hydromagnesite stromatolites (microbialites) up to 7 m high coalesce to form a group of small islands 200 m across. The microbialites are seen to be growing near the mouth of the usually dry Salda River in the southwestern sector. Smaller developments of hydromagnesite encircle the lake and image processing of satellite data reveals a second extensive zone beneath the lake surface over a delta in the southeast. Individual columns a few centimetres high constitute bulbous mounds which are about 2 m in diameter. These columns terminate in domes a centimetre or so across. The domes are often annulated and are covered with a green biofilm a few millimetres thick comprised of cyanobactrial filaments. The columns consist of alternating fine and coarse hydromagnesite layers differentiated on a millimetric scale. The coarser layers near the surface still contain traces of the biofilm.Fossil microbialites were also discovered in the friable hydromagnesite cliffs shoreward of the main developments, though the structures of the individual microbes have not survived. Instead the vestiges of microbialites are easily recognized and delineated by their coarse grain size and high porosity. Annular structures on their upper surfaces can be seen in places. The intervening and overlying material, also comprised of hydromagnesite, is a semi-lithified mud.Bulbous megascopic structures, separated by finer grained magnesium carbonate mudstone, within strata in the 'White Rock', would be strong evidence of a photosynthetic microbial genesis. Another deposit of white rock on the western margin of Juventae Chasma could have a similar origin.