ORE BODIES AND ASSOCIATED IGNEOUS ROOK. 151 



developed on the eastern and western borders. To the eastward 

 at North Dundas, the Five Mile, and Dundas, is a notable develop- 

 ment of basic rocks — gabbros, norites, pyroxenites and peridotites, 

 together with the gabbro-amphibolites, amphibolites and serpen- 

 tines derived from them by secondary processes. The granite 

 porphyry and quartz porphyry of North Dundas form relatively 

 small masses. 



To the westward lies exposed the great granite massif of Mt. 

 Heemskirk, and round its borders are disposed smaller bodies of 

 gabbro-amphibolite and serpentine. 



Of the dykeg^ which penetrate the central portion of the field, 

 only one of basic character has been detected, while there are 

 numerous small intrusions of acidic composition. 



The greater part of the Heemskirk massif is composed of a 

 reddish, coarse-grained granite, which contains both orthoclase 

 and plagioclase felspar and biotite mica. Without any defined 

 boundaries this variety passes over into a paler granite which is 

 characterised by the presence of muscovite, in addition to the 

 minerals named, but more especially by the presence of enormous 

 numbers of nodules composed of quartz and tourmaline, with 

 smaller quantities of felspar and traces of cassiterite. These nodules 

 vary from an inch to six inches in diameter, and stand out in relief 

 upon weathered surfaces of the granite containing them. The 

 granite massif is traversed by numerous dykes or veins of pegmatite, 

 aplite and quartz-tourmaline rock, some of which extend beyond 

 the granite margin into the surrounding rocks. 



The dykes of granite porphyry which traverse the Zeehan field 

 are without doubt apophyses from the magma which produced 

 the granite of Mt. Heemskirk. 



The basic rocks surrounding the granite do not exhibit a great 

 variety of types. The principal masses which have come under 

 geological examination are on the one hand gabbros, in which the 

 pyroxene has given way to massive and fibrous amphibole, or on 

 the other hand serpentines. 



A dyke of an uncommon type — mica-gabbro — has been 

 recognised in one part of the Zeehan district, and side by side with 

 it occurs an aplite of normal character. 



The detailed investigation of the country bordering on the 

 granite has not yet been made. There may be present many other 

 rocks of basic composition. 



To the eastward of Zeehan many different varieties of basic 

 rocks occur which have been penetrated at one point by dykes of 

 granite porphyry^. 



From the study of the field relationships of the members ot 

 both groups the author i* inclined to state the succession and 

 genetic relationship of the igneous rocks as follows : — 



In a deeply seated magmatic reservoir differentiation had 

 already produced a separation into an acidic and a basic magma 

 at a time when the Igneous material was enabled to rise into still 



1 Vide " Geological Map of the Tin Field of North Dundas," Geol. Surv. Tas. Bulletin No. 6, Plate 11 



