Geological Society, 73 



relative position of these bright rays and the dark lines in the laven- 

 der part of the spectrum, which are only visible in the sun's light 

 under the most favourable circumstances, must not be relied on as 

 accurate, since it was determined by measurement, and not by direct 

 comparison. Possible inaccuracy of adjustment will render the 

 fourth place of decimals quite uncertain. The remarkable extension 

 of the spectrum of this electric light, both at the red and violet ends, 

 is, however, indisputable. 



Electric light. 



Sunlight. 





Line X 1*6038 



Bundle of violet rays ... 1*6436 



„ A 1*6069 



„ G 1*6404 



„ K 1*6513 



2nd „ „ 1*6531 (about) 



„ I 1*6548 







„ L 1*6567 



„ „ 1*6574 (about) 









« , 



On Great Fluctuations of Temperature in the Arctic Winter." 

 By J. J. Murphy, Esq. 



GEOLOGICAL SOCIETY. 



[Continued from vol. xxiii. p. 559.") 

 April 16, 1862. — Prof. Ramsay, President, in the Chair. 



The following communications were read : — 



1. " On the Position of the Pteraspis-heds, and on the Sequence 

 of the Strata of the Old Red Sandstone Series, in South Perthshire." 

 By Prof. R. Harkness, F.R.S., F.G.S. 



At the Bridge-of- Allan the lowest beds seen of the Old Red Series 

 are (1) conglomerates of trap-rocks, overlain by (2) grey sandstone 

 passing upwards into red sandstone. These grey sandstones have 

 afforded to Mr. Powrie a Pteraspis (perhaps Pt. rostratus) ; and frag- 

 ments of Cephalaspis have also been found in them. Next above 

 come (3) purple shales (at Craig Arnhall) ; then the brown sand- 

 stones (4) of Downe Castle ; and lastly, at Laurick, grey sandstones 

 (5) again. These all have a N.W. dip, as seen along the Teith. 

 From about Laurick to Callander the strata are best seen in the 

 Keltic burn. They have a S.E. dip, and form the other, but steeper, 

 side of a synclinal trough (about twelve miles wide) ; and here grey 

 sandstone (5), brown sandstone (4) (at Braeklin Linns), purple 

 shale (3), red and grey sandstone (2), and lastly a conglomerate (1) 

 (here composed of felstone) present an analogous succession to that 

 between Laurick and the Bridge -of- Allan. At Callander the con- 

 glomerate lies almost vertically against the metamorphic Lower Si- 

 lurian rocks of the Grampians, trap-rock intervening. The author 

 estimates that these Old Red strata have a thickness of 7000 feet. 



