Geological Society. 311 



The Chair was then taken by Prof. Prestvvich, M.A., E.R.S., 

 Vice-President. 



3. " On Pelanechinus, a new Genus of Sea-urchin from the Coral 

 Rag." By W. Keeping, Esq., B.A. E.G.S., Professor of Geology 

 in the University College of Wales. 



4. " Beinarks on SaurocepJialus, and on the Species which have 

 been referred to that Genus." By E. Tulley Newton, Esq., E.G.S., 

 of H.M. Geological Survey. 



5. " A Microscopical Study of some Huronian Clay-slates." By 

 Dr. Arthur Wichmann. 



Although a considerable amount of attention has been devoted 

 during recent years to the microscopical study of clay-slates and 

 slate-clays, yet in none of the published researches on this subject 

 has any account of the structure of the clay-slates of archaean age 

 been given. The author has availed himself of the extensive series 

 of Huronian clay-slates collected by Major T. V. Brooks in the 

 country around Lake Superior to supply this deficiency. The suc- 

 cession and relation of the rocks described have been fully treated 

 of in the work of Hermann Credner and the publications of the 

 Geological Survey of Michigan. 



The chief object of the author is to discuss the origin of the crys- 

 talline constituents in clay-slates, and at the outset he describes in 

 detail the microscopical character of clay-slate, of novaculite or 

 whetstone, and of carbonaceous shales and slates respectively, dwell- 

 ing more especially on the crystallized minerals which can be detected 

 in each of these rocks, and the nature of the isotropic ground-mass 

 which sometimes surrounds them. He then points out that three 

 theories have been advanced to account for the presence of these 

 crystalline constituents in clay-slates. According to the first of 

 these theories, the crystals in question are regarded as the 

 product of chemical action in the ocean in which the original 

 material was deposited ; the second theory attributes the formation 

 of the crystalline minerals to processes of metamorphism which 

 have taken place subsequently to the solidification of the rocks ; 

 the third theory refers them to aggregative action going on in the 

 still plastic clay-slate mud prior to its solidification. The first of 

 these theories has been maintained by G. 11. Credner ; but against it 

 the author adduces numerous arguments, and especially points out 

 the difficulty of supposing an ocean capable of depositing from its 

 waters at successive periods minerals of such different chemical 

 composition as chlorite, actinolite, &c. In opposition to the second 

 theory, which has received the support of Delesse, the author points 

 out the existence in the rocks in question of broken crystals which 

 have been recemented by the surrounding clay-slate substance. 

 The author is thus led to incline towards the third theory, in favour 

 of which some striking facts, drawn from the microscopical structure 

 of the rocks, have already been adduced by Zirkel. He admits, 

 however, that later metamorphic actions are not to be excluded in 

 seeking to account for the origin of the crystalline constituents of 



