SIR G. S. MACKENZIE on the Formation of Chalcedony. 87 



The term Botryoidal is often applied to mineral forms, and is 

 sufficiently understood. Pendulous I use in preference to Sta- 

 lactitic, because the latter term implies a peculiar mode of for- 

 mation already ascertained *. 



No. 1. The first specimen I exhibit is one in which the agate 

 structure is not visible ; it has a small cavity, lined with minute 

 crystals of quartz, and in its general appearance resembles flint. 



No. 2. This specimen is also massive ; and in its ordinary 

 fracture the agate structure is not seen ; but on the side from 

 which a slice has been taken, that structure displays itself. 

 This specimen is valuable on account of its exhibiting a peculiar 

 fracture, which cannot be described by words. It also shows an 

 exterior shell, such as I have succeeded in separating from ano- 

 ther specimen of the same sort. 



No. 3. This shell has the botryoidal form, and has given the 

 impression of this to the Chalcedony which has filled the cavity 

 formed by it, and which, therefore, is of posterior formation. 1 



No. 4. In this specimen the parallel form is very slight, but 

 sufficiently visible. 



No. 5. This shows the parallel and massive forms combined ; 

 and there is a minute cavity, lined with quartz-crystals, in the 



* This Memoir is printed nearly as it was read to the Society ; and there is un- 

 questionably some awkwardness in the references to the specimens, while neither spe- 

 cimens nor drawings of them can be examined. With respect to drawings, it is ex- 

 ceedingly difficult to represent mineral bodies well, without great labour and expence; 

 and the forms of the specimens referred to in this Memoir, as well as their general ap- 

 pearance, are such as render an attempt to represent them by engraving almost hope- 

 less. Indeed it would be useless, as many of the specimens referred to can be viewed 

 only by turning them in different directions, and placing them in different lights. 

 The facts are stated in such a manner, as, it is hoped, will leave nothing equivocal in 

 the mind of the reader. 



