2l8 



THE BUILDING OF THE DERBYSHIRE LIMESTONE. 



pure massive limestone, here and there of a semi-crystalline 

 character; and below this comes a mixture of more or less 

 thickly-bedded limestone. 



A practical geologist can, in most cases, at once distinguish a 

 limestone, although the different varieties exhibit considerable 

 differences in texture and colour ; but a simple test consists 

 in the application of a drop of hydrochloric acid, when, if there 

 be much carbonate of hme present, vigorous effervescence will 

 take place. When strongly heated, limestone is decomposed into 

 quicklime and carbon dioxide. 



Fig. I. Polished Section of Crinoidal Marble (Wirksworth). 



A common and typical variety of limestone met with in Derby- 

 shire is called crinoidal, encrinital, or entrochal marble. A polished 

 section of this is represented in Figure i. A consideration of the 

 marks seen upon its surface, which are usually white upon 

 a grey background, will lead us to the origin of the rock ; 

 they are the hieroglyphics which, rightly interpreted, enable 

 us to read the history of the rock. It will be obvious 



