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XXIV. 



ON THE DOLOMITE OF HOWTH. By W. F. SMEETH, 

 B.A., B.E. (Petrological Laboratory, Trinity College, Dublin). 

 (Plate III.) 



(Communicated by Professor Sollas.) 



[Eead December 12, 1888.] 



Two patcb.es of dolomite occur in the neighbourhood of Howth. 

 Both are on the coast line, and border on the junction of the moun- 

 tain limestone and Cambrian rocks. They are evidently a portion 

 of the mountain limestone, which has suffered alteration, under 

 the action of solutions containing magnesia. 



The rock is of a light-brown colour, due, as is shown in thin 

 slices under the microscope, to the presence of a brown oxide of 

 iron, mechanically included. The brown colouration does not ex- 

 tend inwards very far from the surface, but ends somewhat 

 abruptly along certain jointings of the unchanged limestone 

 beneath. Along the line of junction blocks of unaltered limestone 

 occur in the midst of the dolomitized portions, and show progressive 

 stages of alteration. These facts point to the introduction of the 

 magnesia in solutions from above. 



The rock was found on analysis to have the following com- 

 position : — 



99-72 



On separating the mineral constituents of the powdered rock 

 by means of Thoulet's solution, dolomite fell between specific 



