66 report — 1859. 



of rocks, it may be advantageously employed as a kind of preliminary quali- 

 tative analysis, in the case of the majority of sedimentary rocks, whether meta- 

 morphic or otherwise, before reducing them to powder, in order to analyse 

 them by the ordinary method. The slow and prolonged action of acids on 

 minerals or rocks composed of a mixture of minerals, or even those mainly com- 

 posed apparently of one mineral, enables us sometimes to discover substances 

 which would otherwise have passed unnoticed, and the constituents of which 

 would consequently be confounded with those of the rest of the rock. 



The number of rocks which resist without decomposition the prolonged 

 action of acids such as HC1 and HO S0 3 at various temperatures, up to their 

 boiling-point, is extremely limited ; and this is especially the case if fragments 

 of rocks be subjected to the alternate action of the two acids. Those espe- 

 cially which have undergone a slight alteration, such as the commencement 

 of the formation of hydrated minerals, always yield to such treatment. 



A great number of rocks, consisting of aluminous silicates or silicates of 

 lime or magnesia, frequently leave, after treatment with acids, skeletons which 

 show us the manner in which many minerals may have been decomposed, 

 the residues which they left often serving as the basis of new formations. 

 In examining calcareous rocks containing such skeletons, it is necessary to 

 use dilute acid solutions, sometimes indeed extremely so ; as concentrated 

 acids might in many instances decompose the skeletons, especially if they 

 appeared to contain hydrated silicates. 



The rocks which I have submitted to examination since the last meeting 

 of the Association may be classified as follows : — 



1. Comparative examination of the residues of Permian magnesian lime- 

 stones from ten localities. 



2. Comparative examination of the magnesian limestone of Howth, Co. of 

 Dublin, contrasted with those of the Permian localities. 



3. Magnesian limestone conglomerate from Downhill, Co. Londonderry. 



4. Examination and analysis of a pseudo-dolomite, found at the junction 

 of the trap and carboniferous limestone, at Stone Park Quarry, 2^ miles 

 north of Six Mile Bridge, Co. of Limerick. 



5. Experiments on the composition and structure of the residues obtained 

 from the Calp or middle limestone, Co. of Dublin, and of the lower limestone 

 shales of Drogheda. 



6. On chloritic slate, and metamorphic limestone derived from it. 



7. On a metamorphic limestone containing garnets reposing on the granite 

 near Gweedore River, Co. Donegal. 



1. Magnesian Limestones from Permian Localities There appears to me 



to be considerable confusion in the minds of some geologists regarding what 

 are called Magnesian Limestones. The terras Dolomite and Magnesian 

 Limestone, in the sense in which they are sometimes employed, seem to 

 imply a similarity in the mode of formation. This is, however, far from 

 being the case. Most limestone rocks, whatever may be their origin, contain 

 some magnesia; and even recent corals and marine shells have been found 

 by the investigations of Dana and Forchammer to contain some. 



I have no intention to propose a nomenclature of magnesian limestones; 

 I merely wish to trace the distinctive characters of some of those rocks 

 by means of the residues which they leave when treated with acids, and 

 which are often the only witnesses which could instruct us as to the mode 

 of their formation. Some of these residues are very characteristic ; thus the 

 Permian are ochrey, and always contain fragments of hyaline quartz, some- 

 times rounded on the angles. Those, on the contrary, derived from mag- 

 nesian limestones formed either by infiltration or in a tranquil medium, and 



