BY W. H. TWELVETREES AXD W. F. PETTERD. D 



'Oranny's property at Lymington. It consists of horn- 

 blende + plagioclase felspar, with the hornblende greatly 

 preponderating. We have placed this dark basaltic look- 

 ing rock among the dioritic aplites called nialchite. 



That this petrographical province extends further north 

 ^Ye have satisfied ourselves by the discovery of blocks of 

 garnetiferous trachyte at the base of the Sugar-loaf Hill, 

 behind Mrs. Cleary's cottage on the road to Cradoc. It is 

 there also associated with Permo-Carboniferous fossiliferous 

 marine beds. It is well known, moreover, that it extends 

 to Oyster Cove in a N.E. direction. 



The word " felspar-porphyry " was applied to the Port 

 Cygnet rocks as a field term, indicating a porphyritic 

 texture. It meant simply that the uniformity of the 

 micro-crystalline ground-mass is interrupted by a profusion 

 of larger crystals which were formed during the intra- 

 telluric period of the history of the rock. The term is 

 only admissible as a temporary expedient for the designa- 

 tion of such rocks prior to definitive examination. It can 

 be discontinued now that the trachytic nature of these 

 rocks is beyond doubt. 



It may be useful to trace the lines along which our 

 enquiry has travelled, and show how they lead up to the 

 results now submitted. 



As a rule, when the colors are white, yellow, grey, we 

 may take it that a lava does not belong to the basic series 

 of rocks. It will be a member either of the acid series, 

 containing over GG per cent, silica, or of the intermediate 

 series with 55 per cent, to GG per cent. As a whole, the 

 Port Cygnet rocks are remarkable for the small quantity 

 of their free silica. They are essentially quartzless rocks. 

 Of course they contain silica in combination, but only 

 sufficient to bring up the SiO^ per cent, to the limit for 

 intermediate rocks, those lying between the acid granites 

 and rhyolites and the basic gabbros, dolerites, and basalts. 

 The silica per cent, corresponds with the specific gravity, 

 which averages 2 • 5 to 2*6. These facts help us in locating 

 the rock in a definite division. Now, in this division the 

 andesites and diorites are characterised l)y plagioclase 

 felspars, while in the trachytes and syenites orthoclase is 

 dominant. In the Port Cygnet rocks orthoclase un- 

 questionably predominates. Trachyte is the volcanic 

 form, syenite the plutonic. 



Haiiy first gave the name of trachyte {rpaxig = rough) 

 to volcanic rocks feeling rough to the touch. In these 

 rocks there is generally very little glass, the ground-mass 

 being more or less crystalline. When they contain plagio- 

 clastic felspar it is an acid variety. When this felspar 



