172 Mr. A. Gages on a Method of Obsei~vation 



as in some thermal springs. No doubt many of those volcanic 

 products have been formed by natural processes similar to that 

 which I have described. 



The opals found in the basaltic district of the North of Ireland, 

 and occurring in the cavities of amygdaloidal greenstone at the 

 Giant's Causeway, have most probably been derived from the 

 gelatinous silica of decomposed zeolites, while some of the spe- 

 cimens of the same locality described as hydrophane, resemble in 

 a remarkable manner certain of the skeletons resulting from the 

 action of acids upon some altered trap-rocks*. 



I may, as an illustration, refer to a variety of magnesite de- 

 rived from the decomposition of basalt, and described by General 

 Portlock in his ' Geological Report on the Counties of London- 

 derry, Tyrone, and Fermanagh,' pp. 114 and 115. This mineral 

 substance presents one of the best illustrations of the peculiar 

 metamorphic changes which occur during the decomposition of 

 some basalts. Dr. Apjohn has given an analysis of the mineral 

 at page 114 of the 'Report ' above named, from which it would 

 appear to be a hydrous silicate of alumina and magnesia. This 

 mineral substance is of a greyish-white colour, and consists of a 

 series of parallel laminse. 



A lamina of this mineral, of about 2 centimetres square by 

 2 millimetres thick, when boiled for some time with hydrochloric 

 acid and then with sulphuric acid, leaves, after an action of several 

 days, a skeleton of amorphous silica, blackened by the siUphuric 

 acid acting upon organic matter derived, doubtless, from the water 

 of infiltration. When the acid and organic matter are removed 

 by washing and ignition, there remains a skeleton of pure amor- 

 phous silica lighter than water, and presenting the perfect form 

 of the primitive substance, visibly constituted of thin laminae su- 

 perimposed like the leaves of a book. After immersion in water 

 for a sufficient length of time, it becomes ti'anslucent, and acquires 

 all the characters of certain varieties of hydrophane. The quan- 



* Common opal occurs, filling the cavities of amygdaloidal greenstone 

 at the Causeway ; Rathlin Island ; Crossreagh, parish of Bally willing ; and 

 in several places along the basaltic range, — principally white, varying from 

 translucent on the edges to opake ; also striped, and sometimes yellowish. 



At Sandy Braes (Connor parish. County of Antrim), it is met in great 

 abinidance, forming strings or irregular veins in the pitchstone porphyry 

 of that district, generally opake and white ; also yellow, or reddish yellow, 

 and highly translucent, having much the aspect of the coarser varieties of 

 amber. Occasionally it presents considerable play of colours : associated 

 with it, but sparingly, jas]3ar-opal is foimd. 



Hydrojjhane is found at the Causeway in small roundish masses in 

 amygdaloid, of a brownish-white colour, much like mountain-cork. Also 

 at Crossreagh, parish of Ballywilling. (Report on the Geology of the 

 County of LondonderiT, Tyrone, and Fermanagh, bv General Portlock, 

 F.R.S.) 



