TRANSACTIONS OF THE SECTIONS. 49 
Considered alone, these numbers would lead to the mere conclusion that the rock was 
a dolomite; but by operating on a moderately sized fragment cut in the direction of 
the fibres, with dilute hydrochloric acid, fer some days, a skeleton remains of asbes- 
tiform tremolite having the following composition :— 
UN CA isle nivale dicen MOS 
Magnesia eer en ene 29 
—a result which shows that the dolomitic rock is a subsequent formation to the tre- 
molite. 
The new mode of conducting the examination of rocks, depending upon the me- 
chanical difference of their preparation, was further advantageously contrasted by the 
author. Some varieties of magnesite Jeave silico-gelatinous residues, by which the 
transition from meerschaum to a replacing pseudomorphite of carbonate of magnesia 
may be followed. Again, crystal of Thomsonite boiled in hydrochloric acid, deposits, 
after removal of the alkali, a gelatinous transparent precipitate, and an opaline skeleton 
remains, presenting the outlines of the primitive crystal, thus exhibiting the stages 
of alteration through which these crystais pass. The opals found in the basaltic di- 
stricts of the North of Ireland have most probably been derived from the gelatinous 
silica of the decomposed zeolites, while the hydrophane of the same locality resem- 
bles in a remarkable manner some skeletons artificially obtained from a laminated 
magnesite derived from the decomposition of basalt, and described by General Port- 
lock in his ‘ Geological Report.’ ‘This mineral, treated by acids, leaves a skeleton of 
pure amorphous silica lighter than water, and having the perfect form of the primi- 
tive substance, consisting of thin laminz superimposed like the leaves of a book. 
After immersion in water for a sufficient length of time, it becomes translucent, and 
acquires all the characters of certain varieties of hydrophane. If left exposed to the 
air for some time, it loses the greater part of its water, but retains a mean quantity of 
about 6°40 per cent., which corresponds with the formula 3Si 03, HO given by Beudant 
for an opake white opal from Castellamonte. 
As an illustration of the decomposition and subsequent reconstruction of rocks, the 
author referred to a pseudomorphite of laminated quartz-rock, derived from magne- 
site, which, though partially crystalline, retained traces of amorphous silica, It would 
appear that the original rock, after the disappearance of some of its constituents, had 
been infiltrated by the silica of alkaline silicates derived from the decomposition of 
trappean rocks. The petrifaction of fossil wood from the vicinity of Lough Neagh 
may be referred to the same class of phenomena. 
Mountain leather and cork from Londonderry, leave, when treated by acid, a light 
spongy skeleton analogous to varieties of nectigue quartz. All these substances are 
evidently the result of a more or less advanced state of alteration ef hornblendic and 
augitic rocks; sometimes the decomposing silicates are entirely replaced by carbonates 
of lime and magnesia, or with carbonate of lime alone, and the original mineral is 
traceable only in the form of a thin superficial film on both sides of the newly-formed 
carbonates. Serpentine, cut into thin laminz and treated by acids, exhibits in a great 
number of instances the original mineral substance from which it has been derived. 
The author concluded by stating that the simple treatment of acids acting upon thin 
laminz was an efficient means of distinguishing true serpentine, which leave a skeleton 
of amorphous silica, from many rocks resembling the former lithologically, 
Ona new variety of Pyro-electric Wavellite. By ALPuoNnsE Gaces, V.R.LA., 
Curator of the Museum of Irish Industry, Dublin. 
The first specimen with which the author became acquainted, was presented to him 
by Mr. G. H. Kinahan of the Geological Survey, and was part of a block found in the 
driff on the banks of White River, four miles south of Loughhill, resting on coal- 
measures. It was accompanied by blocks of limestone, trappean breccia, syenite and 
granite. Wavellite has also been found by Messrs. Jukes and Kinahan in the lower 
bed of the coal-measures, just above the limestone, about three miles north-west of 
Cahirmoyle. 
The present specimen answers in some respects to the description of Fischerite or 
Peganite, as given by Dufrenoy and Dana, 
1858. 4: 
