404 PROCEEDINGS OF SECTION C. 



slightly inclined surface of these Mesozoic sediments. Sub- 

 sequently thfi upper division of the Ipswich coal measures was 

 deposited with slight unconf ormability on the Toowomba lavas, and 

 further overflows of lava took place subsequent to or about the 

 conclusion of the period of deposition of this upper division of the 

 Ipswich 



Members of the Association visiting Brisbane at the next annual 

 meeting will, of course, have every facility for studying the highly 

 interesting volcanic rocks at Brisbane and in its neighborhood, and 

 further evidence may then be adduced to explain the apparently 

 anomalous position of the Brisbane tuffs at the base of a great 

 sedimentary series. 



-o-i^-o- 



10.— THE SYSTEMATIC APPLICATION OF PHOTO- 

 GRAPHY AS AN AID IN MAKING GEO- 

 LOGICAL SURVEYS. 



£1/ E. P. BISHOP. 



0-^-0- 



11.— THE SPECIFIC GRAVITIES OF SOME GEM 

 STONES. 



By A. LIVERSIDGE, 3I.A., E.B.S., Professor of Chemistry, JJinversiti/ 

 of Sydney. 



The following table contains the specific gravities of some gem 

 stones, which were all in the cut and polished condition, except in 

 the cases specified. As the specimens were sufficiently free from 

 flaws and mechanical impurities to be cut and polished for jewellery 

 the specific gravities can be taken as those of pure minerals, 

 and the results should be more satisfactory than those obtained 

 from ordinary cabinet specimens, unless it can be sho^^^l that the 

 specific gravity is altered by the pressure and other treatment they 

 have received during the process of cutting and polishing. 



The specific gravities were taken with special cai-e on an 

 Oertling's best chemical balance by direct weighing, i.e., by 

 .suspending the specimen in a A'ery small and light metal stirrup 

 in distilled water. A specific gravity bottle ^niis found, as is well 



