Vol. 54.] IN THE CAKBOJriFEEOUS LIMESTONE OF DERBYSHIRE. 183 



up the oolitic limestones, destroying both their structures and 

 organisms, and absorbing their impurities, which in some instances 

 retained their original arrangement, but in others became grouped 

 parallel to the outlines of the crystals of dolomite. 



Mr. Strahan expressed his admiration of the slides and spe- 

 cimens exhibited by the Author, and of the conclusive proof which 

 they afforded of his argument. The dolomitization of oolitic Car- 

 boniferous Limestone in South Wales, referred to by the previous 

 speaker, seemed to be due to a similar process of alteration, the 

 replacing mineral in both cases appearing first as perfect isolated 

 crystals, scattered through unaltered limestone regardless of the 

 organic structures. By a gradual increase in numbers such crystals 

 finally replaced the original constituent, and completely obliterated 

 all organic structure. The source of the carbonate of magnesia, 

 however, was no less obscure than that of the silica. Some of the 

 Carboniferous Limestones of North Wales contain doubly-terminated 

 crystals of quartz, but not in such abundance as in the cases now- 

 described. Galena also, in the neighbourhood of a lead-vein, occurs 

 as isolated crystals embedded in solid unaltered limestone. 



The Rev. H. H. Winwood also spoke. 



The Author replied that the microscopic structure of the quartz- 

 rock differed entirely from that of chert ; and that the insoluble 

 residue of the quartzose limestone consisted of quartz-crystals, and 

 contained no chalcedonic or amorphous silica. 



'I 



