452 



Frof. Hull — Structure of Porphyry. 



is probable the chlorite was the first of these minerals infiltrated into 

 the mass of the rock. 



Fig. 2. 



Fig. 2. — Group of orthoclase crystals imbedded in the felspathic matrix, darkened 



by grains of magnetite. Mag. 25 diams. 

 Fig. 3. — Cavity filled with chlorite showing cellular structure. Mag. 55 diam. 



b. Cell in chlorite. Mag. 400 diams. 



Calcite. — The calcite, which is very abundantly diffused through- 

 out the prophyry, occurs chiefly in fissures, often extremely minute, 

 and in the cells, which were probably filled originally with gas or 

 steam. It is always in a crystalline state, and under the microscope 

 the lines corresponding to the cleavage planes of the rhombohedron 

 are clearly developed. With polarized light, the calcite exhibits as 

 usual the faint and delicate pearly or opalescent play of colours, 

 amongst which a pale smalt blue is the most easily recognized. 



The calcite is unquestionably due to infiltration; and its abund- 

 ance may with great probability be attributed to the Carboniferous 

 Limestone of the district, which must once have overspread and 

 covered up the rocks of Lambay and Portraine, which have since 

 been brought to light by the denudation of the limestone. It is easy 

 to conceive that water percolating downwards through the limestone 

 and the thin coating of Old Eed Sandstone beneath would become 

 strongly impregnated with carbonate of lime, which it would deposit 

 amongst the fissures and cells of the older rocks beneath. 



Along with the minerals here described are also to be found 

 crystals of pyrites ; and the order in which the different minerals 

 seem to have been formed is as follows : — First, during consolidation, 

 the crystals of orthoclase; next, the crystalline grains of magnetite; 



