244 PROCEEDINGS OF SECTION C. 



The result would be that certain strata would be charged with 

 phosphate of lime, and others with carbonate of lime, while the inter- 

 vening strata would contain a small proportion of both varieties. As 

 evidence in favour of this theoiy, the Cambrian limestone, calcareous 

 slates and claystones, sandstones, &c., almost invariably contain small 

 percentages of phosphate of lime, as also do the chert and flinty 

 boulders, those found in the limestone generally containing most. 



In conjunction with these occuiTences there is generally a large 

 amount of clay and other argillaceous material also containing segre- 

 gated phosphate, partly due to the erosion of the rocks, sometimes 

 occupying caves and depressions, particularly in the limestone. This 

 formation is of still later date, and the deposit may have been en- 

 riched by the deposition of guano and other animal remains. 



3.— A GEOLOGIST'S SLIDE ETULE, AND SUNDRY PROBLEMS IN 

 PREPARATION OF GEOLOGICAL MAPS AND SECTIONS. 



By W. G. WOOLNOUGH, D.Sc, F.G.S., Acting. Professor- in Geology and Mineralogy, 



University of Sydney. 



In geological survey work, both in the field and in the office, the 

 geologist is called upon to make a number of calculations; and the 

 instrument described in the present communication has been devised 

 to assist in making or checking these calculations. 



The instiimient is easily constructed as folloAvs : — Cut out strips 

 of smooth cardboard — 



22 inches by 3| inches ... ... ... (one piece). 



22 ,, 1^ „ ... ... ... (one piece). 



22 ,, 1| „ ... ... ... (two pieces). 



22 „ 1 „ ... ... ... (three pieces). 



Cut a semi-circular hole about 1 in. diameter out of the middle of 

 each end of the 3i-in. strip. 



Now carefully paste one of the 1-in strips flat along each edge 

 of the 3|-in. strip. (It is well to '" bank up" these strips with pieces 

 of thick paper to allow sufficient " play" for the slide.) 



On top of each of the 1-in. strips paste one of the 1^-in. strips 

 overlapping the narrower underlying one on the inside. By this 

 method a " slide way" is formed (Figs. 1 and 3). 



Take the remaining 1-in. strip and paste it centrally along the 

 1^-in. piece, so that a strip of the latter ^in. wide projects on each 

 side. This forms the " slide" (Figs. 2 and 4). 



Each piece pasted on should be allowed to dry thoroughly under 

 pressure before being covered by another strip. 



The graduation is the next step. Four scales are necessary ; these 

 niay with advantage be increased to eight, but that is not essential. 



