646 DR. J. A. THOMSON ON THE [Dec. 1 9 1 3, 



side of Mount Gledden (Maritana Hill) and in the dump of the 

 Q,ueen-of-the-West Mine, G.M.L. 942 E. Although no natural 

 sections have been observed in situ, large blocks in the dumps show 

 that the pegmatitic rocks occur as narrow bauds in the normal 

 amphibolite. The rock from which they have arisen thus bears the 

 same relation to the main intrusion, as the well-known gabbro- 

 pegmatites of the Lizard district in Cornwall bear to the gabbros 

 of that area. 



Hand-specimens show a very coarse mottled rock, of a prevailing 

 dark-green hue, with white patches representing the felspar and 

 quartz. The most noticeable peculiarities are the platy habit of 

 the femic constituent, and the manner in which these plates are 

 bent into curves with arcs approaching a semicircle, so that a broken 

 surface of the rock seems to have small biscuit-barrels projecting 

 from it. On a typical fragment, less curved than usual, one of these 

 platy crystals measured 4 cms. in length, 1 cm. in breadth, and 2 mm. 

 in thickness. A transverse striation is often to be observed in hand- 

 specimens, although nothing corresponding to it has been detected 

 in section. The quartz-and-felspar or saussuritic aggregates are 

 generally of much smaller size ; but the iron-ores, which have also, 

 a platy habit, attain 5 cms. in their greatest diameters. Veins of 

 yellowish epidote and quartz sometimes traverse the rocks. 



Microscopic study shows that the dark plates consist of a pale-green 

 hornblende, and that the rocks are built on the same model as the 

 normal quartz-dolerite-amphibolites, but on a much grander scale, 

 The chief points of difference are that the pyroxene is represented 

 largely by carbonates, chlorite, epidote, and magnetite, in addition to 

 hornblende, and that the structure is not always clearly ophitic (for 

 the platy felspars are sometimes moulded on the pseudomorphs of 

 the pyroxenes). So far as I am aware, such pegmatites have not 

 been described before, either in a fresh or in an altered state. 



B. The Quartz-Dolerite-Greenstones (Albitized 

 Quartz-Dolerites). 



These rocks, along with those described in the next section, have 

 a relatively-small development in the ' North End,' but occupy the 

 greater part of the quartz-dolerite intrusion in the ' Mile.' The}' 

 are not mere surface-weathering forms of the amphibolites, but 

 exist at depths of 2500 feet and over. Owing to the variety and 

 extent of the alterations that they have undergone, they present 

 little appearance of homogeneity, and the recognition of their 

 original nature is not (at first sight) easy. The rocks here termed 

 ' greenstones '" are more or less dark green from the abundance of 

 chlorite ; but every variation of colour is found between these 

 and white or flesh-coloured rocks, in which chlorite is practically 

 absent. The latter rocks are described in the next section as* 

 bleached greenstones, but it must be clearly understood that the 

 boundary-line between the two divisions is an arbitrary one. In 

 both groups there is every gradation between coarse granitoid 

 varieties, approaching in texture the pegmatitic amphibolites above 



