BY n. D. OSBORNE. 179 



lites and axiolites. Although the green soda-rhyolites are not exposed in the 

 Eailway cutting, they are well developed in portion 154, Parish of Barford, a 

 little to the east, there resting on the dacite just mentioned. 



About 150 feet below the top of the Volcanic Stage, there is a tuff which 

 presents some interesting characters. It is exposed a little to the east of the 

 line of section described above. In thin section it is seen to contain a lot of 

 fragmental quartz, chips of orthoclase and oligoclase, and fragments of a 

 spherulitie rhyolite and pitehstone, the whole being compacted by an iron-stained 

 matrix. This tuff is like many of the other types in the Volcanic Stage in 

 possessing a distinctly felspathic nature. 



The thicknesses for the Martin's Ck. section are given in the following 

 summary : 



Thickness in Feet 



Biotite-quartz-keratophyre 50 



Coarse conglomerate and tuff 300 



Red potash-rhyolite 25 



Soda-felsite . 100 



Toscanite 50 



Conglomerate 15 



Volcanic conglomerate 10 



Fine-grained tuff 15 



Cherty tuff 25 



Dacite 50 



Total Thickness - . 640 Feet 



Preliminary discussion of the Stratigraphy of the Volcanic Stage. 



It is premature to discuss in any detail the variation in the stratigraphy of 

 the Volcanic Stage, as such will only be possible after an exhaustive petro- 

 gTaphical and chemical study of the rocks has been made. The names given to 

 the various units will in all probabilitj' stand after such study, but at 

 present one is not able to say what are the relations between the various types 

 of potash-rhyolite, or between the tuffaceous soda-rhyolites on the one hand and 

 the biotite-ciuartz-keratophyres on the other. 



From the data to hand one can generalise as to the sequence. The only 

 sequence of lavas maintained through the area is the following: hornblende- 

 andesite and andesitic pitehstone (Martin's Ck. type), hypersthene-andesite and 

 andesitic pitehstone, biotite-quartz-keratophyre (Williams River type — in some 

 places one flow, and in some places two), biotite-toseanite — dellenite (Mt. Gilmore 

 type), potash-rhyolites and daeites. This succession may be looked upon as the 

 framework of the sequence, which is modified at various localities by the 

 existence of additional sets of less important flows, respectively more or less 

 peculiar to those localities. Some details with regard to the overlapping of 

 flows from adjacent vents are to hand, and these are withheld until full petro- 

 graphie work is done, but one can state that there is evidence of the former 

 existence of a centre of supply towards the Martin's Ck. end of the area, one 

 possibly near Glenoak and Langlands, and a third in the Mt. Gilmore district. 

 As a result of the position of these, there is a poor development of the Volcanic 

 Stage along the Oakendale Section, and overlapping appears to have occurred 

 at Mt. Gilmore and in the Langlands area. Whether these eruptive centres were 

 of the strictly central type, or existed as a series of local centres along a dominant 

 fissure-line or set of fissure-lines, there is little direct evidence to decide. On 

 account of the uniformity of the petrologieal features and, to some extent, the 

 stratigraphical positions of the various lavas over large tracts of land in the 



