(ieologii of Lihjdide. 361 



Here it is limited l)y tiie (|uart/ite.s. which evidently formed the 

 eastern bank of this channel. It nowhere transgresses the igneous 

 rocks of the Devonian cycle to the east, which prohalily foinicd high 

 giound when the basalt was poured out. 



Macroscopically. it is a dark, non-vc.-jiculai-. very tough, tine- 

 grained rock, with small crystals of g'ccu olivine. 



Microscopically, olivine is abundant in large, regular crystals, 

 and a few large laths of a basic plagioclase are set in a ground 

 mass, composed of felspar laths, lath-shaped augite, granular 

 olivine, and abundant rectangular magnetite, and interstitial 

 glass. 



The reference of this basalt to the older or newer series is by no 

 means easy. Petrologically two criteria are used, viz. : — 



Both tests are inconclusive, but they tend to place the Lilydale 

 basalt in the older series. In the quarry north of Cave Hill, in 

 allot. 301, the basalt is very decomposed. Added to this, the 

 extensive circumdenudation. which is apparent, tends to favour the 

 older age. 



Source.— T'\\\ now. no undoubted vent f)f the older basalt has been 

 found. At Lilydale. in the basaltic plateau, just referred to, there 

 is a crater-like depression breached towards the north-east, which 

 being regarded as a vent Ijy the residents, is therefore known as 

 "Crater Hill." Settioiis on the adjoining roadside resemble 

 tuffaceous deposits. On the opposite side of the road, there is 

 another larger depression opening on the north-west side. In a hole 

 sunk in the bottom of this basin to a depth of about thirty feet, 

 the l)asalt is present all the way. The rock on the rim of the basin 

 has all the appearance of a decomposed fragmental rock. Further 

 evidence is needed, before concluding that the place was an erup- 

 tive centre. 



In conclusion, 1 wish to thank Professor Skeats, who has dis- 

 cussed the problenis witli me from time to time, both in the field 

 and in the laboratory; also Mr. H. J. Grayson, for kindly prepar- 

 ing the micro photogiaphs. and some very thin rock sections; and 

 also Mr. E. J. Dunn, who jici-mitted me to liave four rocks analysed 

 by .Messrs. Uayly and Hall, in tlic gcoIoLMcal Survey's laboratory. 



VII. Summary. 



The following are the most important new results contributed 

 in this paper : — 



I. The district between Lilydale and .Mount Daiidenong has been 

 mapped petrologically for tlie first time, with the exception of the 



