68 report — 1859. 



under the influence of the decomposition of other rocks, contain, ill the 

 majority of cases, crystalline substances in a whole state, or partially decom- 

 posed silicates. 



Having just indicated the comparative distinctive characters of the residues 

 left by the magnesian limestones of different kinds, I will now proceed to 

 describe those of the Permian in detail. 



The magnesian limestones of the Permian group which I have had 

 an opportunity of examining, leave, when treated with hydrochloric acid, 

 more or less abundant residues, offering the same lithological characters. 

 These residues are ferruginous clay, varying in colour from deep red to very 

 pale yellow. These variations of colour are due to the relative proportions 

 of sesquioxide of iron present, and sometimes to that of manganese also. 

 The residues contained besides fragments of transparent quartz, which may 

 be separated by washing. The oolitic characters which some of those mag- 

 nesian limestones assume are always due to those fragments of quartz, which 

 serve as nuclei around which the deposit of carbonates is formed. The quan- 

 tity of residue sometimes exceeds 30 per cent., and often does not amount to 

 | per cent. ; but whatever may be the quantity of the residue, its lithological 

 characters remain always the same. 



The following Table contains the results of my examination of each of the 

 specimens of magnesian limestone from Permian localities. 



No. 3 in the Table illustrates very strikingly the origin of the oolitic 

 structure in calcareous rocks. When a fragment was exposed for a short 

 time to the action of hydrochloric acid, so as to remove part of the lime, 

 the grains of sand were observed standing in a kind of hollow shell. It 

 differs, however, from the generality of oolitic rocks, in which the grain 

 of sand or matter forming the nucleus is surrounded by concentric layers of 

 calcareous matter. In the rock under notice, the grains of sand appear, so 

 far as can be judged by means of a lens, to have been simply imbedded in the 

 cementing parts. 



2. Howth Dolomite. — The dolomite of Howth, Co. of Dublin, belongs 

 to the carboniferous series, and rests on Cambrian slates. It is of a light 

 yellowish-brown colour and has a compact crystalline texture, with many 

 cavities, however, which are filled with well-developed crystals of bitter-spar. 

 When examined with a lens, it appeared to be formed of a series of irregular 

 serrated layers, sometimes containing oxide of manganese in more or less 

 quantity. On being treated with acetic acid, it divided itself into small 

 granular crystals of bitter-spar, resembling an extremely fine sand. It thus 

 presented all the characteristics of true dolomite. 



Treated with dilute hydrochloric acid, it left a residue never exceeding 3 

 per cent., and consisting of a reddish-brown ochrey clay mingled with crystals 

 of quartz, which were separated by agitating the residue in water. They 

 consisted of very fine acicular crystals of opake quartz, having a fibrous 

 arrangement, the edges of some of the crystals being somewhat eaten away. 

 Washed several times with hydrochloric acid, and then treated with hydro- 

 fluoric acid, these crystals yielded an appreciable quantity of alumina, oxide 

 of iron, lime, and magnesia, a circumstance which suggests that they may be 

 the relics of some augitic or hornblendic rock. The Rev. Prof. Haughton, 

 to whom I submitted these crystals, and who examined them, considered 

 them to be " fibrous quartz, and such as occurs in the minute veins of 

 quartz in the slate rocks of which the Hill of Howth is formed." What 

 is most remarkable in connexion with those crystals, is the constancy with 

 which the residue is found disseminated throughout the whole dolomitic 

 mass of Howth. 



