<52 J. A. Dunn: 



■ably accounted for by the almost instantaneous intrusion of the 

 molten material, the magmatic heat being quickly conducted away 

 from the walls. 



Some eight miles south of Bendigo, at Big Hill, the Ordo- 

 ^ncian, at the contact with the Harcourt granitic mass, has been 

 somewhat metamorphosed.^ Typically the sandstones have been 

 altered to a mica horn f els, and the argillaceous sediments to 

 spotted and andalusite slates. For the most part, however, the 

 alteration is rather an induration than an absolute change in 

 the mineral content of the rocks. 



4. Deposition of the Sediments. 



By numerous writers in the past, some of the Ordovician beds 

 of the Bendigo goldfields have been referred to as deposited in 

 shallow water, owing to the common occurrence of ripple-mark- 

 ing.'* It was first thought by Dr. Hall, and later confirmed by 

 T. S. Hart''' that the origin of these pseudo ripple markings is 

 due to the intense compressive forces to which the rocks have 

 l)een subjected. It appears probable that during the process of 

 folding, the resultant stresses along the bedding planes caused 

 -movement of the beds over each other, with the concomitant pro- 

 duction of minute puckers in the more plastic beds. This may 

 be the explanation of the more common occurrence of this 

 pseudo ripple-marking in the slates than in the sandstones of 

 Bendigo. Hence, this evidence of apparent ripple-marking can- 

 not be accepted as a criterion of the shallow-water deposition of 

 the sediments. 



The general fineness in grain of the rocks rather points to the 

 deposition of the sediments some distance from the shore, prob- 

 ably in the relatively deep-water of a continental shelf. The 

 often rapid succession of exceedingly minute bands of slate 

 :and sandstone, with the admixture of occasional quite coarse 

 sandstones, suggests that the sediments were laid down under 

 variable currents, probably a result of tremendous floods wash- 

 ing the material from various sources. 



3. " Report on Ihe Reiidijro Goldfield," No. 1. E. J. Dunn. pas,'e 7. 



4. " Report on tlie Bendijjo Goldffeld," No. 1. K. J. Dunn, paj^c G. 



5. "On some Features of the Ordovician Recks at Daylesford, with a Comparison with Similar 

 Occurrences elsewhere." T. S. Hart, MA., H.C.E., Proc. Roy. Soc. Vic. 1901, page 175. 



