368 Scientific Proceedings, Royal Dublin Society. 



lite in some cases, or asbestus in others. Besides tliese there is 

 in joints and fissures abundance of a green mineral whose com- 

 position approaches that of true serpentine.* 



Pakt III. 



Microscopical Notes on the Appearances of the Serpentine, with 

 Asbestiform Tremolite, from Sligo. 



Four specimens of this remarkable material were sent by 

 Mr. Hardman to me for examination, with the following 

 results :■ — 



No. 1. Dense dark-green serpentine, with a narrow band of 

 nearly white fibrous, silky tremolite.t 



Microscope. — A thin section transverse to the band of tremolite 

 is well seen with an one-inch objective. The serpentinous 

 mass appears as a light yellowish-green amorphous material, 

 with reticulated or net-like divisions, through which are dispersed 

 formless groups of magnetite grains (Plate XXXIII., Fig. 2). 

 The tremolite appears as a band traversing the mass, with 

 fibrous structure, the hair-like prisms being perpendicular to 

 the walls (Plate XXXIII., Fig. 4). It is divided into two 

 portions by a central band of amorphous colourless matter, and 

 it would appear as if the tremolite had crystalised out from the 

 opposite walls of the fissure on either side, leaving a partially 

 unfilled space in the centre, as in the case of some mineral veins. 

 A few translucent spaces occur amongst the mass of the serpentine 

 filled with a colourless material with a wavy fibrous structure, 

 which probably consists also of tremolite. 



Polariscope. — With this the efiect is striking. With crossed 

 nicols the serpentine presents a spangled field of rich saphire or 

 indigo blue, of varying depths, and broken up into individual 

 grains of irregular form. With parallel nicols the same mineral 

 has a slightly yellowish tint. With this vivid polarization it may 

 be inferred that the mass is in a molecularly crystalline condi- 

 tion. 



* Analysis IV. — Green mineral, like chlorite, but. with composition of serpentine— 

 SiOj FeO Al^ O3 Fe^ O3 Mg Water. Total. 

 42-40 2-107 4-80 2-63 32-695 14-69 99-322. 



f In the following description I have called the fibrous mineral of these sections " tre- 

 molite," in deference to Mr. Hardman's determination ; but it does not seem to answer 

 very well in its optical properties to the mineral of this name as described by Eosenbusch. 

 CSee Mich'os. Physlographie^ s. 307). 



