CHEMICAL EXAMINATION OF ROCKS AND MINERALS. 65 



vious explorations, which have extended over several years, has there ever 

 been detected any trace of an ichthyolite — fish-scale, fin-spine, or tooth — a 

 noticeable fact, considering the highly fossiliferous character of the strata, 

 and the indications that many of them give of littoral as well as of deep-sea 

 conditions of deposit. 



Looking at the palaeontological value of Mr. Slimon's discoveries, and the 

 additional interest they have conferred on Palaeozoic Geology, your Com- 

 mittee would respectfully urge upon him a continuance of his labours, con- 

 joined with the hope that, if at all compatible with the other requirements 

 of the Association, a further grant of say a610 or 3620 should be made to 

 assist in so desirable an object. 



Report on the Results obtained by the Mechanico -Chemical Examination of 



Rocks and Minerals. By Alphonse Gages, M.R.I. A., Curator of the 



Museum of Irish Industry. 

 I had the honour of bringing before the Section at the last meeting of the 

 Association at Leeds, a short paper entitled " On a Method of observation 

 applied to the study of Melamorphic Rocks, and on some Molecular Changes 

 exhibited by the action of Acids upon them." The principal feature of this 

 method of examination consisted in exposing thin plates of rocks, or crystals 

 cut in certain directions, to the slow action of solutions of acids or alkalies 

 of different degrees of concentration, under such varied circumstances as the 

 special characters of each rock may suggest. The general result of this 

 action was the gradual removal of some or of all the bases, a residue being left, 

 the structure and composition of which indicated the mode of formation of 

 the original rock. 



The idea of submitting rocks or minerals to the action of various solvents 

 is not new. But hitherto experimenters have operated upon the powdered 

 mineral. I operate upon fragments which exhibit not only the chemical 

 constitution of the substance under examination, but what is in many in- 

 stances of still greater importance, the mechanical constitution also. An 

 example will explain still better the difference. 



If we powder a piece of alum and put it into water, it will dissolve, and so 

 far as the appearance presented by the powdered mass, uniformly. But if 

 we take the same piece of alum, and instead of breaking it up, grind a flat 

 surface upon it, and place it, as Daniel did, in water with its polished face 

 downwards, the water will act upon that face very unequally ; after a time 

 crystals will stand out in relief, and what looked like a homogeneous cry- 

 stalline mass, will be shown to be made up of a confused mass of interlaced 

 crystals cemented together. 



Observers have no doubt dissolved minerals in fragments as well as in 

 powder, but they have not, so far as I am aware, done so with the object of 

 studying the peculiar mechanical arrangement of the components of rocks, 

 and certainly have not done so as a methodical system of examination. 



If, as Daniel's experiments show, we may learn much regarding the 

 molecular structure of even a crystalline mass of a homogeneous substance 

 by the manner of dissolving it, how much more so must this be the case with 

 such complex mechanical mixtures as most rocks are ! Before detailing the 

 experiments which I have made during the past year, I may observe, that 

 although the application of this method of examination (which, for want of a 

 better word, we may call niechanico-chemical) is limited to a certain number 



1859. F 



