40 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. [Mra0mS (K ,o xix, 



where the serpentine belt is unusually wide (with the exception of the region of maximum width). 

 The smaller masses of gabbro cut sharply through the serpentine in irregular directions, but 

 so far as the limited information goes, the larger masses of gabbro form broad gneissic inter- 

 calations lying more or less concordantly within the serpentine belt. But here detailed mapping 

 is wanting. The gabbros are largely saussuritised, olivine gabbro is rare, and the eucrite 

 type is the most abundant. The writer's analysis of the least altered specimen showed it to 

 consist of anorthite-bytownite and chromiferous diallage in which the enstatite-molecule was 

 richly present (Benson '13). The gabbro-veins of the smaller masses have sometimes a very 

 coarse pegmatitic-ophitic structure. As a result of secondary changes, veins of grossularite 

 have been produced. Judd ('95) also noted these rocks. 



Several parallel but shorter bands of serpentine are known to lie east of the main belt, but 

 they have been little investigated. An interesting structural problem is afforded by the occur- 

 rence of masses of serpentine as at Port Macquarie directed to the northeast (Came '97) . The 

 writer has suggested that these result from virgations of the strike of the main axis of folding 

 and serpentine-intrusions, and that the serpentine is not (as has been stated) essentially a 

 roughly elliptical peripheral zone about a central complex of granite batholiths (Benson '18a). 



Other small patches occur farther to the northeast on the Clarence River, but are less well 

 known. They seem to be sill-like masses, invading highly crushed Devonian (?) phyllites, 

 etc., parallel to the general north-northwesterly strike. 



Very different from these is a small mass of essexite of roughly lopolithic form and pre- 

 sumably Tertiary age, invading the horizontally bedded Triassic mudstones at Prospect near 

 Sydney, described by Jevons ('11) and others. It is suggested that here the spreading of the 

 magma from the central feeding channel was controlled by the varying thickness of the cover- 

 ing strata, probably not more than 1,000 feet, due to the surface topography. 



Nothing definite has been published with regard to the serpentines of Queensland. They 

 are known to occur at Pine Mountain near Ipswich, an isolated patch striking to the northwest, 

 but completely surrounded by younger Mesozoic sandstones (Cameron '99) ; they form a long 

 broad band running in the same direction west of Gympie, and another similarly placed but 

 longer band east of Rockhampton. Both of these are considered by Mr. Dunstan, Chief Gov- 

 ernment Geologist for Queensland, to lie for the most part concordantly within Devonian strata 

 which are more or less metamorphosed. The Gympie mass of serpentine has been invaded 

 by granite, which forms also a batholith farther to the west but adjacent to the serpentine. 2 

 Other isolated occurrences have been reported but not yet described. 



In Queensland the latest folding is more modern than elsewhere in the continent, even 

 Cretaceous rocks being involved (Andrews '16) but in southern New Guinea, Tertiary rocks 

 even as late as Pliocene have been folded and thrust toward Australia (David '14a). 3 This 

 is the first of the arcs of folded ranges of Tertiary age that surround the Australian 

 nucleus. Its northern margin exposes the arc of ancient schist, but strongly folded Mesozoic 

 rocks occur, the strike of which may be traced west into Bum and Ceram, where Creta- 

 ceous rocks are greatly involved. In these islands basic eruptive rocks occur, and these are 

 traced in isolated occurrences of gabbro and serpentine along the north coast of New Guinea, 

 both in Dutch New Guinea and the Finnisterre Range. This is continued into the Louisiade 

 Islands to the southeast of New Guinea. Stanley ('12, '15, '21) reported serpentines, etc., in the 

 metamorphic sediments of the Owen Stanley Range and Woodlark Island, and found a long 

 sill-like mass of gabbro in the schists and gneiss of Misima (St. Aignan), the easternmost of 

 the Louisiade group, its general strike being very slightly south of east. In the Solomon 

 Islands, belonging to an outer arc, numerous fragments of peridotite and gabbro have been 

 found of which some details have been cited by Suess (IV pp. 311-312). Mention may here be 

 made of the occurrence of gabbro in Tahiti, at the base of a varied complex of foyaitic rocks, 

 and very basic basalts. The relationship of the gabbro to the other rocks does not seem quite 

 clear yet (Lacroix '10, Marshall '14). 



■ Private communication. 



a These strongly folded rocks are now considered pre-B. Miocene. (Ct. Benson Trans. N.Z. Inst. IV. pp. 115.) 



