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. 



