Igneous Pebbles. 28i> 



matrix, resembling decomposed igneous material. The pebbles 

 appear to be chiefly volcanic, but occasionally coarsely crystalline 

 rocks, plutonic and metamorphic in character, are seen. Accord- 

 ing to Mr. Herman! diorite pebbles are predominant; but in my 

 limited examination I came across no such pebbles. Microscopical 

 examination of thin sections of these rocks shows that many of 

 them are closely related to one another; and in the specimens col- 

 lected by the author, all stages are represented between horn- 

 blende diabase, and the spherulitic facies of the same rock, or 

 variolite. No acid igneous rocks, either plutonic or volcanic,. 

 Avere found. 



Section near the steel bridge over the Thomson river, about txvo^ 

 miles north of its junction with Stringer's Crpplc'—The river, which 

 has followed the strike of the sediments for some considerable dis- 

 tance below here, at this point, cuts across the strike, and good 

 sections are available, especially so on the east bank of the river 

 Avhere cuttings made by the Long Tunnel Company for a tram 

 track, have uncovered the sediments. 



Fossiliferous grits and calcareous beds w^ith occasional patches 

 of limestone occur at intermediate points between Cooper's Creek 

 and the steel bridge, and are again present further north of this 

 point. These outcrops are all in east dipping beds, and are on 

 the same line of strike as the Copper Mine series of limestones 

 and conglomerates, and are undoubtedly the northern continua- 

 tion of the same beds. 



The section, near the steel bridge, shows that the beds consist 

 of hard, indurated black and grey, fine grained sandstones and 

 shales, interbedded with well cleaved slates, and coarse grit bands 

 and conglomerate; all dipping east at 50° to 70o. The beds 

 throughout the section are calcareous, and locally contain highly 

 fossiliferous lenticles and nodules of limestone. A fairly gradual 

 passage can be traced from gritty sandstones into coarse fossili- 

 ferous grit and conglomerate. The grits are often honeycombed 

 owing to the removal of the fossils. In the more calcareous beds 

 the fossils are preserved in their original calcareous matrix. 

 Corals (Favosites chiefly) and crinoids appear to be the most com- 

 mon fossils in these beds. 



The band of conglomerate is well seen on the north side of a 

 small creek entering the Thomson River from the east. Pebbles 

 of quartzite are most common in this conglomerate, and igneous: 



i Op. cit., p. 12. 



