Rocks near Heathcote. 303 



10. — The Age and Relations of the Rocks of 

 Heathcote — 



a — Fossils of the Dinesus Beds - - 340 

 /> — Fossils of the Ordovician Rocks - 340 



^— The Age of tiie Black Cherts - - 341 

 d — The Age of the Diabase and other 



Igneous Rocks . . . . 341 



e — Physical Geography of the Lower 



Ordovician Period - . - - 343 



y — Correlation of the Heathcote Rocks 



with other areas - - - - 344 



11. — Conclusion -------- 345 



1. — Introduction. 



Victorian geolog}' abounds with problems for the solution of 

 which conflicting hypotheses have been proposed by different 

 observeirs. Among these controverted questions none is of 

 greater interest and importance than the one wnth which this 

 paper is concerned, and few present greater difficulties in the way 

 of a completely satisfactory' solution. 



The problem may be stated briefly to be the origin and age 

 of certain basic and acid igneous rocks, their relations to cherty 

 and jasperoid series, which generally accompany them, and the 

 relations of all the above to the Silurian and to the Ordovician 

 rocks of the district. 



The work of mapping the area presents considerable difficulty, 

 as over a considerable part of the district rock-junctions are 

 hard to find, and when found the relations of the rocks to one 

 another are not easy to interpret. Furthermore both igneous 

 and sedimentary rocks have in places been so altered that it is 

 not surprising that different observers have interpreted the 

 geology in different ways. Microscopic examination of the rocks 

 helps considerably in following the interesting series of meta- 

 morphio and metasomatic changes which the rocks have under- 

 gone, but partly as a consequence of these changes some of the 

 questions which ordinarily an appeal to the microscope would 

 have settled, remain to some extent unsolved. 



