304 THE DESERT SANDSTONE. 



ages of the volcanic rocks and the so-called Desert Sandstones, 

 for both as yet have been imperfectly surveyed. There are many 

 areas of volcanic rock, such as basalts, diorites, and other igneous 

 or trap formations in the Northern Territory ; but if we regard 

 the magnesite as an ash deposit, it is not easy to say as yet to 

 what portion of the volcanic history they owe their origin. 



The uppermost magnesite strata form a rock which is very 

 much decomposed. They are seldom uniform for any great 

 extent either in colour or material ; pure white, croam-colour, 

 mottled, and various shades of purple and red prevailing in ever 

 varying tints. There are few marks of stratification, but long 

 divisional lines which indicate protracted periods of rest in their 

 accumulation. If we accept the volcanic origin we may suppose 

 that the craters or trap rocks connected with such deposits must 

 have been very rich in magnesia, the most probable source of 

 which would be olivine. About ten miles north of the Katherine 

 River there is an area of volcanic rocks, the limits of which I was 

 not able to examine. In the bed of a creek near which I had 

 formed my camp there was an appearance of trap rocks, amongst 

 which there was a basalt very rich in olivine. It cannot be said 

 exactly, however, from whence the magnesite proceeded. It may 

 be due to some such rock as suggested. The deposit is too 

 extensive to have been derived from freshwater action on the 

 underlying rocks which are rich in mica, and probably other 

 magnesian minerals. A marine origin is of course out of the 

 question. 



The volcanic deposits which are found on the Katherine River 

 are not the only ones in Arnheim's Land. A large area occurs to 

 the west of Port Darwin, and a very large volcanic district is 

 found at the head of the Victoria and of the Fitzmaurice Rivers. 

 The rocks here exposed are of modern character and probably 

 belong to several distinct periods, certainly to two, of which 

 there is constant evidence in the continent of Australia. I have 

 mentioned a significant fact connected with the strata as far as 

 my observations extend ; namely, that wherever they are developed 

 trap-rocks are associated with them. If it will be borne in mind 

 that I am not extending these observations beyond the limits of 

 my own experience, I might add that the converse of this 

 proposition is true, that is wherever there are volcanic rocks there 

 are extensive accumulations of volcanic sand ; though what I am 

 presuming to be ash deposits are not always presented in the form 

 of magnesite. 



It is not easily understood why these magnesite deposits have 

 been preserved so extensively in North Australia, and are to be seen, 

 rather rarely, in connection with the Tertiary trap rocks elsewhere. 

 Circumstances, we may presume, have combined for their 



