Geological Society. 175 



the relation P = ??rE.I the stresses due to bending and 

 stretching. 



Again, taking the case where a = 11 ins., ?c = *007 ton per 

 inch run, E = 13500, and m = *l, we obtain : — 



t = -305 in. 

 Max. stress due to bending = 16*86 tons/in. 2 



„ „ stretching = 1*05 „ 



Maximum tensile stress =17*91 ,, 



By varying the value of m other values of t and stress 

 may be obtained, from which data a curve showing the 

 " variation of stress with plate thickness" may be constructed 

 as indicated by B in the diagram, fig. 3. 



The curve A — drawn for comparison — in the same diagram 

 is reproduced from the previous paper. 



Taking an allowed stress of 10 tons/in. 2 , the corresponding 

 thickness of plating is *418 in. as compared with *376 in. in 

 the previous case. 



XV. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from vol. xxxi. p. 572.] 



April 5th, 1916.— Dr. Alfred Harker, E.K.S., President, 

 in the Chair. 



r PHE following communication was read : — 



• The Picrite-Teschenite Sill of Lugar (Ayrshire) and its Differ- 

 entiation.' By George Walter Tyrrell, A.R.C.Sc., F.Gr.S. 



This sill occurs near the village of Lugar in East Central Ayr- 

 shire, and is magnificently exposed in the gorges of the Bellow and 

 Glennmir Waters, just ahove the confluence of these streams to form 

 the Lugar Water. It has a thickness estimated at 140 feet, and 

 is intrusive into sandstones of the ' Millstone Grit.' The contacts, 

 consist of a curiously-streaked and contorted basaltic rock, passing 

 at both margins into teschenite. The upper teschenite, however, 

 becomes richer in analcite downwards, and ends abruptly at a sharp 

 junction with fine-grained theralite. The lower teschenite becomes, 

 somewhat richer in olivine upwards, but passes rapidly into horn- 

 blende-peridotite. The central unit of the sill is a graded mass 

 beginning with theralite at the top and passing gradually into- 



na 



