260 ACCOUNT OF THE MINERALOGY 



conclusion I did not find to be justified by the observations 

 which occurred to us ; opal and the mixture of zeolite and 

 calcedony having uniformly very different aspects. 



The first specimen of this description which I procu- 

 red, is a very interesting one. The outer surface, or that 

 which lay next the matrix, is formed of radiated zeolite, 

 which has also been projected into the cavity, in the form of 

 stalactites. The whole of this is covered over with calcedony, 

 which is so intimately blended with the acicular fibres of the 

 zeolite, that it is nearly impossible to trace the demarcation 

 between the substances. And, again, upon this calcedony, is 

 imposed a coating of most slender stalactitic fibres of the same 

 material. In the centre of the zeolitic stalactites, there is a 

 point, like that occupied by green earth, in those of calce- 

 dony, from which the zeolite diverges. Here the zeolite 

 is infinitely harder than is usual with the varieties of that 

 substance, — an effect no doubt produced by its intimate con- 

 nection with the calcedony. 



In cavities of this description, we sometimes found quartz 

 and sometimes zeolite ; but the latter does not, like the for- 

 mer, uniformly occupy the entire surface, being occasionally 

 dispersed in solitary crystals, among the stalactites of calcedo- 

 ny, and sometimes, though rarely, accompanied with carbonate 

 of lime. 



The only variety of crystallized zeolite which occurred in 

 these cavities, was Apophyllite, presenting two or three very 

 beautiful varieties of crystallization : 



1. In rectangular prisms, terminated at each end by faces set 

 at right angles to the axis, and deeply truncated on all 

 the solid angles, so as to form a truncated pyramid, 



the 



