INTRODUCTION. 33 



by strong heat, which usually causes the sections to crack and lose their 

 colour, and often become opaque and of a whitish hue. These effects are 

 produced also in nature in places where the deposits have been traversed 

 by intrusive rocks. Bleached wood-opals of this kind are common in the 

 neighbourhood of the Siebengebirge and of the eruptive trachyte of Ober- 

 cassel and Tokay 1 . Exposure to rain sometimes completely removes the 

 petrifying material from fossils in calc-spar, and then nothing is left in the 

 stone but the cavity which held the fossil, and in it a few remains of organic 

 substance. The wall of the cavity is often covered with crystals of calc- 

 spar. The well-known nodules of the refuse-heaps at the copper mines 

 of Ilmenau, which contain fishes and small branches of Conifers, have most 

 of them unfortunately suffered from this mode of destruction 2 . 



Goppert (1) and (17). 2 Solms, Graf zu (1). 



I) 



