258 ACCOUNT OF THE MINERALOGY 



dark green, with a tinge of blue, to a pale green, and even a 

 light straw-yellow. It is always opake. It is not easy to 

 conceive, that this substance could have existed without some- 

 thing to support its very delicate fibres ; but yet this appears 

 to be quite evident, from the examination of specimens, and 

 consequently adds one more difficulty to the solution of the 

 formation of Calcedony. 



When calcedony occurs in hollow cavities, they are simply 

 mamillated, ornamented with stalactites, or lined with crys- 

 tals. Externally, they are of a very fantastic shape ; but with- 

 in, the arrangement is more symmetrical. The upper part 

 Jbrms a kind of dome, with a smooth mamillated surface, or 

 appended all over with stalactites, which generally hang per- 

 pendicularly, but sometimes ramify in -every direction ; while 

 the lower part of the cavity is filled up, so as to present an 

 horizontal surface or platform, affording a pleasing contrast 

 with the irregularity opposed to it. 



This platform is evidently the result of circumstances sub- 

 sequent to the formation of the stalactites, as they not only 

 occasionally penetrate it,but sometimes the horizontal part seems 

 to have risen to the top, so as to have filled the cavity, and en- 

 velope the -stalactites entirely. This part of the mass also 

 varies from the rest, by presenting different shades of colour, 

 forming the onyx, or Band Calcedony of the lapidaries ; it is 

 often accompanied with semi-opal, white, yellow, and green in 

 colour, and also with the opake white substance, known by the 

 name of Cacholong. It likewise often happens, that the upper 

 and stalactitic part of the cavity is covered over with a second 

 deposition, forming a distinct coat, which sometimes differs 

 in colour. Besides the circular arrangement which this pro- 

 duces in the section of the stalactites, when they are cut across 



and 



