330 SECTIONAL TRANSACTIONS.— C. 



River Trent in north Lincolnshire. These measures appear to have been 

 laid down in a broad geosyncline with the maximum depression taking 

 place along the line of the present Pennine axis. 



The Productive Measures are fully represented to the east of the present 

 workings in the Yorkshire Coalfield, but there is a marked attenuation of 

 each of the main subdivisions in this direction. This is also accompanied 

 on the whole by a pronounced deterioration of the more important coals. 

 The maximum productiveness of the Yorkshire Coalfield probably lies 

 along or close to the present outcrop of the Main Productive Measures. 

 The geological evidence at present available suggests that any optimistic 

 estimate of the potential resources of the concealed coalfield to the east 

 should be accepted with considerable reserve. 



Mrs. S. W. Alty. — Secondary crystallisation of tourmaline in Lower 

 Devonian sediments in Michigan, U.S.A. 



Grains of tourmaline showing secondary crystallisation have been 

 observed in numerous samples of Lower Devonian rocks from oil-wells in 

 the Lower Peninsula of Michigan. The secondary tourmaline occurs as 

 a colourless margin attached to well-rounded tourmaline grains. The latter 

 are invariably coloured, brown, yellow, pink, etc., and strongly pleochroic. 

 The colourless authigenic portion is in complete optical continuity with 

 the original grain ; it always occurs at one end of the c-axis of the grain, 

 and often shows striations on it parallel to this direction. 



The original tourmalines are all well rounded and were clearly deposited 

 in the sediment before secondary crystallisation occurred. That this 

 process took place within the sediment is plainly evident from the fact that 

 the secondary margin has a sharp edge which has not suffered any abrasion. 



The rocks containing these tourmaline grains vary from arenaceous to 

 dolomitic, and often contain anhydrite and sometimes oil or brine. They 

 belong to the Sylvania division of the Lower Devonian, and form part of 

 a synclinal basin of Palaeozoic sediments. There is no sign of volcanic 

 activity in the region. 



Dr. A. Raistrick. — Correlation of the Glacial Retreat stages in the north of 

 England. 



Mr. J. A. BuTTERFiELD and Prof. A. Gilligan. — The Conglomerates 

 underlying the Carboniferous Limestone of the Sedbergh and Tebay 

 districts. 



Conglomerates below the Carboniferous Limestone occur in many isolated 

 patches in the N.W. of England, and are especially well developed and 

 exposed in the river valleys of the Sedbergh and Tebay areas. Settlebeck 

 Gill, Sedbergh, affords a most complete section. The lowest deposits, 

 seen resting on the Silurian rocks, consist of fine-grained Sandstones, which 

 at the top contain lenticles of conglomerates, and then supervenes a thick 

 bed of coarse conglomerate. As the beck is ascended the deposits occur 

 in the reverse order, finishing with red sandstone. In the Tebay area the 

 sections in the conglomerates show a complete passage through coarse 

 red conglomerates, red sandstones, green and grey conglomerates and 

 sandstones (including plant-bearing shales), and brown mudstones into the 

 Carboniferous Limestone. In this area is an interesting local phase of 

 green monogenetic conglomerate occurring as an alluvial fan in the red 

 deposits. 



