Geology of Broad meadows. 171 



Previous References. 



The rock at C is described on the quarter sheet as a ''cherty 

 limestone too hard and impure to be profita.bly burnt for lime/' 

 and "' of recent tertiary age," and, further, that it contains 

 '■ brackish-water shells (TruncateUa filosa) and living species." 

 Reference is made to it by A. R. C. Selwyn (8) in his " Descrip- 

 tive Catalogwe of Rocks and Minerals,"' among the older pliocene 

 rocks. He calls it an impure limestone which probably contains 

 much magnesia, and was formerly burnt for lime. He states 

 that it occurs underlying the newer basalt, with tertiary quartz- 

 ites and ferruginous grits. He also notes that it contains fossil 

 shells (TruncateUa), and an analysis (quoted 'below by J. C. 

 Newbery) is recorded. The same rock is again referred to by G. 

 H. F. Ulrich, F.G.S. (9), under the heading, " Dolomite (Mag- 

 nesian Limestone)," when the above statement of the occurrence 

 is repeated, Avith the addition that in the bed of the creek, 

 below the outcrop of the band, occurs a similar rock that 

 encloses TruncateUa, and is most likely derived from the 

 former. 



Geological Relations and Origin. 



Along the small gully at C, the limestone, outcropping at a 

 loAver level than the newer basalt, does give the appearance of 

 a bed underlying the newer basalt. The included boulders of 

 basalt, however, clearly show that the bed must be younger 

 than the newer basalt, and the outcrop can only be considered 

 as a superficial plaster on the side of the gully extending but a 

 few feet inwards, and not, therefore, to be associated with the 

 tertiary quartzites and grits which do underlie the newer basalt. 



All the evidence, variability of composition, lack of bedding 

 and continuity, and the presence of lime-encrusted boulders, 

 points to a chemical rather than a mechanical origin. This 

 chemical origin may be ascribed to magmatic waters and rain 

 waters which, percolating through the basalt, have dissolved 

 out some of the lime and mag-nesia and re-deposited it along 

 the prominent joint planes, and these have determined the direc- 

 tion of the streams. Hence we should expect the limestone to 



