136 H. S. SuTYiTiiers : 



cipal mineralogical difference is the presence of the hypersthene in the 

 dacite, and its absence in the granodiorite. 



In most cases the groundmass of the dacites is holocrystalline, but:, 

 in two places, viz., at Cheniston, near Upper Macedon, and at Hesket^. 



SiO^ - - 62-54 - - 64-04 



Al^O^ - - 16-66 - - 15-58 



PeV - - 1-04 - - 0-80 



Feb ^ - - 5-54 - - 4-47 



MgO - - 2-68 - - 2-64 



CaO - - 3-92 - - 3-52 



Na.,0 - - 2-66 - - 2-42 



K.,6 - - 2-47 - - 2-80 



h'o+ - - 0-46 - - 2-25 



H"0— - - 0-17 - - 0-38 



CO^ - - nil - - • nil 



Tid^ - - 1-20 - - 0-80 



P O^ - - 0-20 - - 0-18 



MnO - - tr. - - tr. 



Li^O - - tr. - - ir. 



CI? - - tr. - - tr. 



99-54 99-88 



I. Dacite 50 yards south of Braemar House (Clyde) Stables. 

 II. Granodiorite, near Braemar House (Clyde). 



are types with fine grained groundmass, consisting of devitrified glass.. 

 These types occur near the outer margin of the base of the dacite 

 dome, and are practically in contact with the Ordovician sediments, 

 and so may be taken to represent portion of the original chilled mar- 

 gin. Sections of these two rocks show the presence of the ordinary 

 phenocrysts, so that it seems certain that the phenocrysts had deve- 

 loped prior to extrusion. With the exception of biotite, the phenocrysts 

 are of the high temperature dry fusion type. In the normal dacite 

 the biotite occurs most commonly as aggregates bordering the hypers- 

 thene crystals. Near the contact between the dacite and granodiorite 

 the first sign of contact metamorphism is the production of more mica 

 at the expense of the hypersthene, until near the junction of the two 

 rocks the hypersthene is seen to be completely replaced by a mixture 

 of biotite and quartz. The biotite has been formed by reaction be- 

 tween hypersthene and the alkali felspar molecules in the groundmass. 



The Hesket and Cheniston types are practically free from biotite 

 so that it may be inferred that the temperature of the magma at 

 the time of extrusion was rather higher than the reaction temperature 

 between hypersthene and alkali felspar, but that in part the cooling 

 after extrusion was sufficiently slow for some reaction to take place. 



In the Strathbogi6 Ranges, and the Tolmie Highlands, the effusive 

 type is better described as quartz porphyrite, being rather more acid 

 and distinctly coarser in grain than the typical dacite. The ground- 

 mass has the same granulitic texture, and the phenocrysts are labra- 



