194 pre>*ident's addres^s — section c. 



MIDDLE DEVONIAN. 



The Buchan Series. — The Felsites. 



In the Buchan district as stated above, Howitt (43, 48), has 

 shown that in phices a mixed series comes between the Snowy River 

 porphyries and tlie Middle Devonian Buchan Kmestones. This mixed 

 series Howitt describes as the Lower Buchan Beds and the limestones 

 as tlie Upper Buchan Beds. The lower series generally consists of 

 calcareous tuffs, felsitic tuffs, breccias, and conglomerates with inter- 

 calated felsite flows. Howitt links this series with the Middle 

 Devonian limestones since it shows in several places a gradual 

 passage from volcanic material up into pure limestone. He separates 

 it, however, from the Snowy lliver porphyries for several reasons. The 

 porphyries are a sub-aerial series, while the Lower Buchan series he 

 regards as subaqueous. The presence of water-worn conglomerates, 

 consisting partly of igneous and partly of L. Pakieozoic sedimentary 

 rocks, indicates different conditions of deposit from those of the 

 fragmental rocks of the Snowy River porphyries. In some places the 

 junction of the two series is bounded by a fault. The thickness of 

 this series has been estimated by Howitt at 750-1,000 ft. 



Petrograpliical and Clieniiral CJiaracfers. 



A specimen from Gellingal (described by Howitt) may be taken 

 as illustrating the character of one of the intercalated lava flows. 



The ground mass is yellow, felsitic, and shows fluidal structure 

 in waving bands of irregiilar width. Felspar prisms including some 

 plagioclase, irregular and defined quartz crystals, a little magnetite 

 and a chlorite pseudomorph jirobably after hornblende occur in 

 the ground mass. No name was given to the rock by Howitt, but 

 the description suggests tliat it should be grouped with the por- 

 phyritic rhyolites. Most of the rocks of this group are of fragmental 

 origin even when appearing compact in the field. An example from 

 Butcher's Creek showed under the microscope that a large number of 

 irregular felsitic fragments are arranged in bands and that these, 

 together with fragmentary crystals of quartz and orthoclase, are set 

 in a compact felsitic base. The rock was probably a banded tuft' 

 which has been indurated possibly by secondary silicification. No 

 chemical analysis of any of the lavas has been made. From the 

 calcireous tuft' at the base of the Buchan limestone, however, Howitt 

 selected a sample, dissolved out the carbonates, and made an analysis 

 of the residue in order to arrive at a conclusion as to the character 

 of the igneous fragments in the rock. The result of the analysis 

 here given indicates its derivation from fragments of a rhyolite- 

 quartz-porpliyry. 



SiO„ = 79-62 K2O = 3-94 



ALO, = 9-99 Na„0 = 2-64 



Fe^O, = 3-22 



CaO = -37 Total =100-00* 

 MgO = -22 



* The total amounted to 101*58, from which the above was calculated. 



