207 



Microscopic Examination of Supposed 



Volcanic Dust from the Northern 



Territory. 



By W. G. Woolnough, B.Sc, F.G.S. 



[Read August 4, 1903.] 



The material is very fine in grain, so that microscopic exami- 

 jaation is difficult. It is of such a liBterogeneous character 

 that detailed description is almost impossible. The most 

 abundanit detierminabl© constituents are long organic fibres 

 with relatively bright polansation colors. Next in abundance 

 come sipicules of various shapes. — anchor-shaped, dumb-bell- 

 shaped, and so oin. These consist of .silica, and are quite 

 isotropic. They are derived, probably, from holothurians and 

 sponges. Fragments of foraminifera (Globigerina ?) can be 

 detected. Each chamber gives the characteristic cross be- 

 tween crossed nicols. The truly volcanic material is sub- 

 ordinate to the other constituents. 



Felspar in perfectly glassy granules are moderately plentiful. 

 It shows no trace of twinning, so' far as I have been able 

 to determine. Some sections are bounded by cleavages more 

 iOT less marked. The refractive index is in all cases higher 

 than that of xylol balsam. 



Magnetite is fairly considerable in amount, and includes some 

 of the largest fragments. It is somewhat remarkable that 

 this, the miuieral of highest specific gravity, should be present 

 in such comparatively large fragmentsi. 



Much dark brown glass is observable. It is filled with 

 minute hair-like microlites, which are faintly doubly refrac- 

 ting. There is a small amount of pyroxene. Whether this 

 is rhombic or monoclinic I cannot detennine, as the frag- 

 ments are very minute. A faint greenish-yellow color can be 

 detected, and the double refraiCtion is noticeably of a^ higher 

 order than felspar. 



An occasional piece of hornblende or biotite completes the 

 listu 



It has been suggested that this dust has been derived from 

 "the great West Indian eruptions of 1902. I am inclined to 

 ihink that we must seek a nearer source for it. The abund- 

 ance of magnetite has already been commented on. I think 

 it im^proba-ble that such, comparatively, large fragments of this 

 Tnin,eral, which possesses a specific gravity of 5.17, could drift 

 lialf round the earth. 



