422 GEOLOGY AXD PETKOLOGY OF THE GREAT SE.^rEXTIXE BELT OF X.S.TT., 



(4) the present writer took the view that possibly the hornblende andesite might 

 represent the original undifferentiated magma, but a thorough chemieal investiga- 

 tion of the whole series is needed belcire any definite opinion can be formed. 



CosiPARLSox With Other Carboxiferous Areas ix the State. 



The Currabubula rocks show many analogies with those of other Carboniferous 

 areas further to the south. One of the localities whose rocks have been petrol- 

 ogieally studied in any detail is Pokolbin (52), and many points of similarity 

 are noticed with the rocks under discussion, although the sequence of eruption ap- 

 pears to have been different, a point which, however, inay not be of any great 

 significance . 



A re-study of the slides of some of the Pokolbin locks reveals a closer rela- 

 tionship to the Currabubula types than tlie published descriptions would indicate. 

 For example, though Mingaye's analysis proves the existence of a strongly potassie 

 rhyoUte, yet slides of other rhyolites show a dominance of albite among the pheuo- 

 crysts, linking these rocks with the soda rhyolites of the Kuttung Series. 



The Pokolbin rocks described as trachj-tes also contain considerable albite in 

 addition to the orthoclase (indeed the soda-felspai- occasionally predominates) and 

 although there are slight textural differences, these do not obscure the similarity to 

 the quartz keratophyres of Currabubula. The chemical similarity of the ande- 

 sitie pitchstones from the two localities has already been commented on. In 

 addition, certain of the Pokolbin andesites are pilotaxitic and are strikinglj' simi- 

 lar to those from Warragundi, except for the almost complete absence of pyroxene 

 phenocrysts . 



The present writer has recently been privileged to examine a series of rocks 

 collected by Mr. C. A. Sussmilch during his investigation, iji conjunction with 

 Professor David, of the Carboniferous sections at Paterson, Seaham, Clarence- 

 town and Eelah , (4) . The series comprises soda rhyolites, toscanites, dellenites, 

 dacites, homlilende andesites, hornblende mica andesites and pyroxene andesites 

 and pitchstones. The hornblende andesite of the ]Vlartin's Creek type and the 

 pyroxene andesite and pitclistone are absolutely identical with the corresponding 

 Currabubula rocks . The soda rhyolites usually contain biotite, but some of them 

 are quite <"omparable with the Currabubula rocks. There is, however, a complete 

 absence of tracliytoid keratophyres in the southern area, if we except the trachytic 

 fragments present in the tuffs. whiU^ the extensive series of toscanites, dellenites, 

 and dacites is wanting at Currabubula. It may be, of coui'se, that these two 

 groups are chemically equivalent. Nothing more basic than pyroxene andesite 

 lias been discovered by David and Sussmilch among the rocks definitely known 

 to be Cai'boniferous in the area examined by them. Dr. "Walkom and the pre- 

 sent writer, however, considered that the natrolite basalt of Pokolbin. which was 

 regarded as forming contemporaneous flows in tlie Lower ^larine conglomerate, 

 might rejiresent the la.st phase of the vulcanicity initiated in Carboniferous times 

 (52, p. 304) and Professor David has recently suggested that the Werrie basalt 

 may be the equivalent of the natrolite basalts which are so extensively developed 

 in tlie Hnnter River Valley at the base of the Permo-Carl)oniferous (41 . 



A noteworthy fact in connection with the Currabubula area is the great im- 

 portance and extent of the intrusive rocks in it as contrasted witli the soutliern 

 area-s, where intrusions are comparatively insignificant. Tlie elucidation of the 

 causes of tliis <lifference belongs to tectonic geology rat1i(M- than fo ]ietrnlogy, and 



