A HISTORY OF DERBYSHIRE well represented. Amongst the mollusca are numerous lamellibranchs, gasteropods and cephalopods. A few species of fish have also been found. Some beds appear to have originated from reef-like accumulations, like those of existing coral reefs and shell beds, others are more or less fragmental and formed of broken corals, crinoid stems and brachiopods and other shells which have been spread out on the sea floor. The fossils vary very much in the amount of attrition they ihave undergone. They are often very well preserved, so that the convolutions and spiral bases are clearly marked. Near Castleton is a conglomeratic limestone made up almost entirely of water-worn shells arranged with their flat surfaces nearly parallel to the bedding planes. In other parts of the district the lime- stone has a granular and sometimes an oolitic structure, and in some cases contains pebbles of a previously consolidated limestone. Bands or way-boards of clay and also thin partings of shale occur frequently amongst some of the massive limestones. They are only local and soon thin out, so that they cannot be identified over any extent of country. In several places a thin bed of impure coal has been found some way down in the beds of limestone. These phenomena indicate that though some of the limestones were formed in water comparatively at rest and free from mechanical sediment, that others were formed in moving water which sometimes contained a large quantity of sediment. Many limestone beds are apparently free from fossils, but when examined with a lens are found to contain numer- ous foraminifera or to have a crystalline structure with few if any traces of fossils. Many of these crystalline limestones originally contained fossils which have now become obliterated by subsequent alteration of the rocks, by dolomitization, silicification or contact metamorphism due to the intrusion of igneous rocks. Dolomitized limestone, in which part of the carbonate of lime is replaced by a double carbonate of lime and magnesia, is locally known as dunstone. It forms castellated outlines, such as those at Harboro' Rocks and on the slopes of the valley between Longcliffe and Bradbourne Mill. It weathers with a rough and gritty surface which is often pitted with small holes. Silicified limestone or quartz rock is found in many localities. Its microscopical structure and its relation to the surrounding rocks prove it to be a limestone which has been entirely converted into crystalline silica. The gradual passage may be traced from an ordinary limestone with few if any crystals of quartz through a quartzose limestone into a quartz rock of crystalline structure. The marmorized limestone, which has a saccharoidal or sugary appearance, is sometimes found near intrusive rocks. Fine examples may be seen near Peak Forest and in Tideswell Dale, in the one case above and in the other below an intrusive igneous rock. The carbonate of lime has been rendered crystalline by the molten rock.
