334 Ernest W. Skeats : 



were ooniposed of diabase fragments, wea-e of such composition 

 and character that chemical interchange most readily took place, 

 resulting in the formation of various kinds of silicious diabase, 

 and of the different tj'^jeis of cherts ass;ociated Avith the sedi- 

 ments. Even in these rocks of basic composition the replace- 

 ment has taken place in varying amounts in di£Eea-ent places, and 

 this may possibly be due to differences in chemical composition 

 or to physical differences in different parts of the series, such 

 as original differences in texture or in the porosity of the rock 

 through which the solutions passed. 



Evidtna' from Tatong and Lancefield Areas. — The selective 

 character of the silicification is perhaps more strikingly illus- 

 trated from other areas. Mr. Summers (op. cit.) has shown 

 that near Tatong cherts are interbedded with normal sediments. 

 A visit which I recently made to the Mt. William and Lancefield 

 districts provides another illustration. In the quariy from 

 which the typical Lower Ordovician Lancefield graptolites have 

 been obtained, unaltered graptolite bearing shales are clearly 

 interbedded with silicified shales containing gi-aptolites, and 

 with dense cherts in which no organisms are visible. In both 

 these cases it seems to me to be clear that the alteration of the 

 rocks cannot be due to contact m,etamorphism by diabase in- 

 trusions, for some of the shales are quite unaltered. The selec- 

 tive silicification must, I think, be connected with original 

 chemical and physical conditions in the beds themselves. 



The only evidence I have seen in the field of typical contact 

 alteration of the Ordovician rocks near the diabase series is at 

 Mt. William, a short distance from Mr. Donaldson's house, and 

 I am indebted to that gentleman for showing me the principal 

 rock outcrops. Near here, besides the Ordovician rocks there 

 occur a dense diabase, a granite porphyry intrusive into the 

 diabase, and an outcrop of a coarse-grained granitic rock which 

 ia an extension of the granitic rock of the Cohaw Ranges. Tlic 

 granite-porphyry is intrusive into the diabase as the micro- 

 granite of Heathoote is intrusive into tlie dial)ase there. I 

 think it has probably orginated in the same way a.s a residual 

 more acid part of the diabase magma. It has caused little or 

 no visible alteration in tho diabase, and is probably of com- 

 paratively superficial origin. The coarsc\trrained granitic rock, 



