Volcanic Necks at Anderson's Inlet. 167 



DiFFERKXCK BETWEEN STRUCTURE OF THE VoLCAXIC NeCKS. 



The cause of the difference in structure between the necks in 

 the Juiubunna hill country, and those in the Anderson's Inlet 

 and Powlett Plains series is, apparently, due to a difference in 

 altitude. Those in the former district, which, without observed 

 exception, are composed of basalt, and have no agglomerate or 

 tuff, represent probably the lower portions of volcanic necks ; 

 while those of the plains series consisting of tuffs only, or of tuffs 

 and agglomerate, or of tuffs, agglomerate and basalt, are probably 

 the higher portions of similar contemporaneous necks. Ages of 

 denudation have in the former case worn away the upper portions 

 of the necks in the Jumbunna series, together with the contigu- 

 ous Jui-assic strata, and all of the younger sediments, if any, 

 which overlay thena. In the latter case, owing, probably, to 

 protective covering and lower altitude, this upper portion of the 

 necks has been preserved, and is only now undergoing denudation. 



The conclusion is then forced upon one that the plains series 

 were at one time at a greater altitude than the former. What 

 then accounts for the reverse difference in present altitude ? 

 Faulting seems to be the cause, and faulting on a large scale. In 

 dealing with an area such as this of the plains, covered as it is 

 with a thin series of clays, sands, gravels and pebbly drift, and 

 devoid of natural or artificial sections, it is impossible to obtain 

 anything that serves as a definite stratigraphical guide or datum ; 

 and in the absence of the occurrence of similar deposits in the 

 Jumbunna hill country one has to turn to the Jurassics them- 

 selves in the hope of finding evidence to assist in arriving at a 

 conclusion. 



A careful examination of every channel along the western and 

 northern side of the Powlett A^alley to the junction of Lance 

 Creek with the Powlett has proved that in almost the whole of 

 the cases in which dips have been obtainable, or the strata 

 observable, they have shown an abnormally high angle of dip for 

 Jurassic strata. The strata themselves have in several places 

 been greatly compressed, crushed and tilted, and are so extremely 

 like the highly-inclined strata of Silurian areas, both in structure 

 and composition, as at first sight to raise doubts of their belonging 

 to the Jurassic system. Dips of over 70 deg. are frequent, and 



