Dr. W. Mackie — Specific Gravities, 503 



to the presence of floating ice during its formation, and indicates 

 a beginning of glacial conditions even before the commencement of 

 the elevation. 



In Ballycroneen Bay a section of more than usual interest is 

 exposed. The ' head/ which here rests immediately on the rock- 

 ledge, is overlain by two distinct Boulder-clays. The lower af these 

 contains shell- fragments, chalk flints, and boulders of Wexford and 

 Waterford rocks, and is obviously the Boulder-clay of the Irish Sea 

 ice. The upper is the ordinary local Boulder-clay of the district 

 laid down by the ice which moved from west to east over Cork. 

 The beach is therefore prior to both these ice-flows. 



The postglacial raised beach is also represented in Cork Harbour, 

 and is quite distinct from the preglacial beach. It consists for the 

 most part of ledges of estuarine clay lying in sheltered spots and 

 raised two feet or so above high-water mark. 



The relative succession of events for which evidence has been 

 obtained appears to have been as follows : — 



1. Land higher than at present — erosion of valleys now 

 submerged. 



2. Land depressed to about eight or ten feet below present level — 

 formation of preglacial raised beach. 



3. Elevation of land — accumulation of blown sand, and sub- 

 sequently of lower ' head.' 



4. Advance of the Irish Sea ice from the east at least as far 

 as Power Head, and deposition of marly Boulder-clay, followed 

 by advance of ice from west Cork and deposition of upper ' local ' 

 Boulder-clay. 



5. Accumulation of upper ' head ' — land at some time about three 

 feet lower than at present, deposition of estuarine clays now above 

 high- water mark. 



Finally, we would call attention to the complete similarity of 

 the preglacial beach to that of Gower in South Wales, described 

 by Mr. Tiddeman. The only difference worthy of notice is the 

 comparative scarcity of fossils in the Cork beach — a difference 

 which is easily accounted for by the quantity of spring water which 

 issues along the platform through the gravel and sand of the beach. 



VII. — A Eapid and Easy Method of Estimating Specific 



Gravities. 



By William Mackie, M.A., M.D. 



THE following rapid and for most purposes sufficiently accurate 

 method of estimating specific gravities of rocks, minerals, 

 sands, etc., will recommend itself to those familiar with the apparatus 

 of the chemical laboratory. 



The substance, if a rock or mineral, is first broken into coarse 

 fragments under ^ inch in diameter. A quantity of the fragments 

 is then accurately weighed in a porcelain crucible or other handy 

 vessel. Say this weight is W. An ordinary burette graduated in 

 'ro c.c. is filled with water to a certain height, say, X c.c. (which is 



