286 



and the norites is one of texture. The norites are granitic this rock 

 is trachytie. 



Between Scoggieside Farm and the Tay Bridge is a remarkable volcanic 

 breccia. The dominant rock of this breccia is red in colour and has a 

 specific gravity of 2'58. It is composed of a compact hornstone-like 

 ground-mass in which lie porphyritic felspars, probably oligoclase, and a few 

 crystals of biotite. Under the microscope the ground-mass is seen to be 

 much decomposed. Traces of flow structure may, however, be detected and 

 microlites of felspar are common. The rock appears to be a mica-oligoclase- 

 porphyrite. Mr. DURHAM speaks of it as a dacite, but Prof. JUDD does not 

 mention the occurrence of quartz among the porphyritic constituents. 

 Quartz does, however, occur in the " porphyry " of Lentretheu in Forfarshire 

 and Prof. JUDD states that this " porphyry " is allied to the rock in question. 

 Associated with the mica-oligoclase-porphyrite is a remarkable vitreous 

 rock. In its unaltered condition this is of a dark grey almost black colour 

 and possesses a resinous or sub- vitreous lustre. Its specific gravity is about 

 2'31 and its silica percentage 67'2l. The porphyritic constituents are 

 plagioclase and biotite. The plagioclase crystals are often broken ; the 

 detached fragments of one individual often lying near each other in the 

 same slide. The biotite is characterized by intense pleochroism and is 

 probably similar to that of the Lentrethen dacite hereafter to be described. 

 The ground-mass is composed of a glassy base, felspar microlites (probably 

 orthoclase), trichites and globulites. The base shows the most exquisite 

 perlitic structure. Prof JUDD describes this remarkable rock as a mica- 

 dacite glass. He found that portions Avhen heated before the blow-pipe 

 swelled up in "caulirlo wer-like excrescences till they attained a bulk at least 

 eight or ten times that of the original fragments." The resulting product 

 was found to be a white pumice which floated on water. 



The vitreous rock decomposes in the field into a white powder in which 

 the felspar crystals remain intact. This powder loses 12*4 p. c. of its weight 

 when dried at 110 C. and another 1O1 p. c. on ignition. It is regarded by 

 Prof. JUDD as bearing the same relation to the acid glasses as palagonite 

 does to the basic glasses. Mr. DURHAM shows that the vitreous rock occurs 

 in nests in the mica-oligoclase porphyrite. In the majority of cases these 

 nests are now simply represented by hollows containing more or less of the 

 white powdery substance above referred to. It is only under exceptionally 

 favourable circumstances that the unaltered dacite glass can be obtained. 



As the "porphyry "of Lentrethen has been referred to and as the dominant 

 felspar of this rock appears to be plagioclase it may be described in this 

 connection, though in chemical composition it is in all probability an acid 

 rock. The specimen on which the following remarks are based was given 

 to the author by Mr. RUTLEY. The rock is of a bright red colour. Glassy 

 felspar, quartz, and both white and black micas (the latter largely pre- 

 dominating) may be easily recognized with a hand lens. The ground-mass 

 gives the colour to the rock. In general appearance the specimen closely 

 resembles many rhyolites. Under the microscope the felspars are seen to 

 be perfectly fresh and very free from inclusions. They frequently show 



