70 REPORT — 1859. 



4. Pseudo-dolomite. — This rock was found, according to Mr. O'Kelly of 

 the Irish Geological Survey, at the junction of the trap and carboniferous 

 limestone at Stone Park, 2\ miles north of Six Mile Bridge, Co. of Limerick. 

 It presents the appearance, at first sight, of the dolomitic limestone of Howth 

 and other carboniferous localities; and isof a brown colour passing into yellow, 

 being traversed by a great number of fine veins of calcite. It is covered by 

 an ochrey substance, similar to that which results from the decomposition 

 of the trap. It left, after digestion with dilute hydrochloric acid, a residue 

 preserving the form of the piece of rock ; it had, however, so little cohesion, 

 that it separated into grains on the slightest agitation. 



The composition of this rock may be represented thus : — 



Carbonate of lime 57*620 



Carbonate of magnesia 5*892 



Carbonate of iron 7*590 



Alumina 0*590 



Water 2*820 



Felspathic residue 25*780 



100*292 



The residue, when washed and dried, exhibits under the microscope cellular 

 fragments of an apple-green colour, analogous to some residues derived from 

 the decomposition of felstones and greenstones. With this substance were 

 also found grains of hyaline quartz. 



This residue is attacked by boiling sulphuric acid, which leaves the quartz 

 debris untouched. Analysed in this way the following results were ob- 

 tained : — 



Silica and fragments of quartz 73*491 



Alumina 9*467 



Fe 2 3 4*127 



Lime 0*552 



Magnesia traces 



Potash 4-277 



Soda 1*451 



Water 6*502 



99*867 



It results from this analysis, that the residue is a felspathic mass, 

 disintegrated by some mechanical means before it became enveloped by the 

 calcareous matter which forms the existing rock. Unless this were so, the 

 quartz could not be found in a fragmentary condition. 



If the rocks which formed the subject of the preceding observations were 

 analysed in the ordinary way, by crushing them to powder, all the evidence 

 regarding their origin and probable mode of formation, which has been so 

 well exhibited by slowly operating with acids upon fragments, in such a 

 manner as not to break up or alter the foreign substances enclosed by the 

 carbonates, would have been obliterated. Where dolomitic rocks are asso- 

 ciated with basalt and other igneous rocks, and enclose silicated minerals, 

 such as tourmaline, tremolite, &c, which are characteristic of igneous rocks, 

 geologists are able to recognize at once the connexion between the dolomites 

 and the igneous rocks; but in very many cases dolomitic rocks bear no such 

 visible evidence of relationship to other rocks, and yet many have been 

 formed by their metamorphosis. The long list of constituents, such as 



