-COLLECTION AND PRESERVATION OF FOSSILS 351 



graptolite lies, and in the end the hidden part may be 

 broken before it is exposed. If, however, we press the 

 knife-blade vertically downwards along a line parallel to 

 the hidden part but a quarter-inch or more away, the 

 strain on the concealing laminae of shale will very likely 

 cause them to split off just where wanted. 



Brushes, the bristles though stiff being softer than 

 most fossils, can be applied directly to the surface of 

 fossils. The dental drill used by dentists for the filing 

 of teeth can be adapted to the cleaning of fossils: it 

 enables a rapidly rotating brush to be applied to any 

 point on the surface of a fossil, with much more con- 

 centrated and rapid effect than that of a hand-brush. 

 Wire brushes can even be used to clean pyritic fossils 

 embedded in hard slates. 



Where mechanical means fail, chemical methods may 

 be used. The simplest of these is ordinary weathering, 

 which in many cases disintegrates matrix without injur- 

 ing fossils (though in other cases the reverse is the case). 

 Very weak acids may effect the same result more rapidly. 

 In general acids must be resorted to with great care, 

 since most fossils are calcareous and at once attacked by 

 them. Where it is desired to expose more fully a fossil 

 partly embedded in tough limestone, the exposed surface 

 may be varnished to protect it and acid poured on : 

 careful watch must be kept so that as soon as any more 

 is exposed it may be washed and varnished. 



A generally safer reagent is caustic potash. This is 

 applied dry, small pieces being scattered over the surface 

 of matrix which it is desired to remove : as the potash 

 deliquesces it penetrates into the matrix and disintegrates 

 it, while it does no harm to the fossil if it be an echino- 

 derm or a mollusc. (It must not be used, however, for 

 brachiopods, their laminated shell being easily penetrated 

 and flaking away in consequence.) The fossil is then 



