Ordovician Mocks. b'S 



merit and plotting by strike, dip, and pitch, and if it were always 

 possible to employ this method, palaeontoUigy, as it is used here, 

 would lose much of its value. Anyone who has worked over a con- 

 siderable area of country where exposed sections are few, knows 

 how really limited, under such circumstances, the mathematical 

 method is, and, where sections do occur, how interdependent the 

 mathematical factors are. In the constantly varying angle of dip 

 according to the portion of an anticline or syncline exposed at the 

 •surface, anything like accuracy is impossil)le and with the long 

 low curves of pitch one is at a complete loss. Added to these diffi- 

 culties, there is at Castlemaine the problem of overturned beds 

 Avhich occur in the east of the area and render valueless observa- 

 tions of dip obtained in shallow cuttings. These are often vitiated 

 also by surface drag or warp diagonal to the directions of dip and 

 strike, which gives a false dip. 



V. Stpatigraphical Value of Graptolites. 



In the slates of Castlemaine there is sufficient evidence to be 

 obtained of the life history of many species of graptolites to afford 

 a fairly complete set of zonal fossils. The evidence is cumulative 

 and not isolated, and that of tlie many sipecies that together make 

 up a facies is rarely at fault. From 'a zonal standpoint Dr. HalU 

 has used the Dichograptidae — Tefragrapfus approxi/nafus (Nich.), 

 T. friiticosus (J. Hall), and Didymogra2)tus hifidus (J. Hall) — with 

 conspicuous success in his classification of the Lower Ordovician 

 rocks of Victoria. Miss EUes^, regarding the rise and fall of a! 

 genus .and species, observes that " a certain resemblance of tliecal 

 chawacteristics, number of thecae in a given space, inclination of 

 ventral and apertural margins to the axis of the stipe, and the 

 amount of thecal overlap may be regarded as (a) of genetic origin 

 and therefore (b) of systematic importance; and further, that a 

 natural group with relatively few stipes Avas evidently developed 

 from multiramous forms, so that of the usually accepted classifi- 

 cation the Dichograptidae are highly important chronologically." 

 There seems no doubt that the gradual progression from multira- 

 mous to simpler forms is world wide. At Castlemaine Clonograptus 

 is common in the lower beds, and Tetragrapttu and Goniograptus 

 are more common in the lower than in the higher beds. Diplograp- 

 ius occurs infrequently in the middle beds, but 1>ecomes more com- 



1 Recent advances of our knowledge of Victorian Graptolites and elsewhere. 



2 Graptolite Fauna of the Skiddaw Slates. (J. J.G.S., vol. 54 (1898), p. 529 ff. 



