ALUM SHALE. 133 



numbers on the beach usually preserve their colour and their solidity, 

 the sea water preventing their decomposition. Within the last ten 

 years, the nodules have been collected, at some of the alum works, to 

 be employed in the manufacture of terras, or Roman cement. For this 

 purpose, they are burnt in a kiln ; then broken, and pounded with 

 stampers, and lastly ground to powder in a mill. This powder, when 

 mixed up properly with water, forms a durable cement. The nodules 

 that contain much pyrites are rejected as unfit for this use, the more 

 calcareous nodules being the best. 



Many of the nodules contain calc spar, which often occurs among 

 the petrifactions ; the chambers of the ammonites and nautilites being 

 usually filled with it. Siliceous crystals are not uncommon in similar 

 situations, the cavities in the nodules being sometimes lined with them. 

 Several of the nodules are of the kind called septaria, being traversed 

 by septa, or cross veins, dividing them into compartments. These 

 for the most part abound with calc spar. 



In the cavities of the septaria, and other nodules, we sometimes 

 meet with petroleum, or mineral oil. When first exposed, it is gene- 

 rally quite fluid, and of the colour of oil ; but it soon becomes viscid 

 and black, and at last hardens into a kind of pitch, which readily 

 melts with heat, and when ignited burns Avith a crackling noise, and 

 emits a strong bituminous smell. It is occasionally found in the viscid, 

 or in the hardened state, at its first exposure. As this substance is usu- 

 ally, if not always, accompanied with animal remains, being most com- 

 monly found in the chambers of nautilites and ammonites, it may be 

 questioned, whether the animal oil, belonging to the shellfish, may 

 not have contributed to its formation. 



Some of the larger nodvUes or blocks, of a rounded or oval 

 shape, are incrusted with a calcareous substance, which has been 

 termed cone-in-cone coralloid. It is composed of an immense number 

 of cones, from an inch to six inches in height, all pointing towards 

 the central block, having the interstices between them filled with 



