[ 139 ] 



XXII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 71.] 



May 22, 1889.— W. T. Blanford, LL.D., F.R.S., President, 

 in the Chair. 



'TVELE following communications were read : — 



-*■ 1. " Notes on the Hornblende Schists and Banded Crystalline 



Hocks of the Lizard." By Major-Gen. C. A. M c Mahon, F.G.S. 



The Lizard district has been visited by the author on three occa- 

 sions during the years 1887-8-9, and the specimens of the rocks 

 collected were subjected to microscopic examination. After sum- 

 marizing the work of previous writers, the author proceeded to 

 consider the hornblende schists. He described these rocks and 

 gave a table showing their constituent minerals. He noted the 

 absence of quartz, the presence of pyroxene, and the fact that the 

 minerals present are those commonly met with in volcanic rocks 

 either as original minerals or as secondary products, and he con- 

 siders that the microscopic study of the schists confirms the 

 opinion of some previous writers that the schists had a volcanic 

 origin and consisted principally of ash-beds. The absence of free 

 quartz militates strongly against the supposition that they were 

 originally sedimentary rocks of an ordinary character, whilst 

 the fact of their being bedded shows that they are not plutonic. 

 The author has found no evidence that the foliation of these rocks 

 is due to dynamic deformation, and gives reasons for supposing that 

 such was not the case. The rock seems to have been originally 

 homogeneous, and its banding produced at a later stage by the 

 segregation of the hornblende in planes parallel to the bedding. 



The rocks furnish abundant evidence of the action of water, as 

 shown by the presence of calcite, chlorite, steatite, and other pro- 

 ducts of aqueous action, as well as by channels fringed with magne- 

 tite, ferrite, or limonite. The action of water in converting augite 

 into hornblende may be distinctly traced when the slices still contain 

 pyroxene. The production of periodical currents of water through 

 the water-bearing strata adjoining the roots of a volcano was com- 

 mented on, and the author suggested that the banding of the horn- 

 blende schists was produced by such water leeching out unstable 

 minerals, such as pyroxene, from the spaces between the planes of 

 lamination, and the formation of comparatively stable minerals, 

 such as hornblende, along those planes. The Lizard rocks contain 

 good examples of the formation of hornblende in the wet way, that 

 mineral having been deposited in cracks in such a way as to join 

 together the ends of hornblende crystals severed by these cracks. 



The " granulitic " group, of which the author gave a table 

 showing the constituent minerals, was then described. Judged by 



