Geology of N illahcootie. 297 



the east, as we find a series of shore-line conglomerates occurring 

 along the eastei-n flank of the Tatong Hills and continuous with 

 these we get the silurian conglomerates which overlie the phos- 

 phate beds at Mansfield. The sedimentary series have been much 

 folded and contorted, and have been intruded by the granites, 

 and allied rocks. This intrusion is clearly proved by the well- 

 marked contact metamorphism exhibited in the sedimentary 

 rocks near the junction of the two series. The presence of acid 

 veins from the granite intruded into the silurian rocks, gives 

 further evidence as to the relation of the granite to the sedi- 

 ment an" rocks. 



This fixes the age of the gi'anites and porphyries as post- 

 silurian, that is if the silurian age of the sediments be accepted. 

 There is no nece.ssity, however, to depend solely on the contact 

 in this area, as the rocks of the Strathbogies certainly belong 

 to the same series, and there is no doubt that they are intruded 

 into the silurian, as contact metamorphism is very marked all 

 along the southern boundary' of tho Strathbogie massif. The 

 southern extension of the porphp'ies forming the Toombullup 

 and Tolmie Ranges is covered by sandstones of lower carbnni- 

 ferous age, and there is no sign of contact alteration in these 

 sandstones, so that the porphyries are undoubtedly pre-carboni- 

 ferous, and consequently belong to some part of the devonian. 



The lower devonian was a time of intense earth movement 

 in Victoria, and these earth movements were accompanied by 

 intrusions of igneous rocks, so that there is little doubt that 

 the granites, granite porphyries and quartz porphjTies of this 

 area belong to the lower devonian. 



Tho relation of the igneous rocks to one another is exceed- 

 ingly interesting. The mineral composition of the various types 

 of igneous rocks in the area shows that they have been derived 

 from a common magma,, in which very little differentiation has 

 taken place. From Mount Samaria to the Broken River we .get 

 an ideal section illustrating the effect of pressure on a con- 

 solidating magma, the increase of coarseness of grain of the 

 constituent minerals being to a certain extent proportional to 

 the increase of pressure under which the minerals crystallized. 

 Commencing at Mount Samaria with a. quartz porphyry having 

 an exceedingly fine groundmass, we pass to the porphyry at Wild 



