66 DK. A. R. DWEEEYHOTJSE ON INTRUSIVE EOCKS [Feb. 1909, 



quantity of oligoelase and orthoclase. Biotite is represented 

 sparingly by decomposition -products. Muscovite is also present in 

 small quantity. 



jSos. 80, 80 a, & 80 b were all collected within a length of 

 20 yards, along a line at right angles to the outcrop. 



Specimen 81. — This specimen shows the actual junction of 

 the granite with a highly altered andesitic lava. The acid rock 

 has the appearance and texture of a quartz-porphyry, while the 

 Borrowdale lava is dark grey in colour and consists largely of 

 secondary mica. 



The granite is similar in type to No. 80, but somewhat finer in 

 texture and containing a considerable amount of felspar. It also 

 contains phenocrysts of quartz and small nests of decomposed 

 biotite, with a few phenocrysts of oligoelase. The quartz-pheno- 

 crysts show strain-shadows, and the rock resembles a quartz- 

 porphyry with a microgranitic base. 



The Borrowdale rock consists of a very fine mosaic of quartz, 

 with abundant minute crystals of secondary brown mica. There 

 are also traces of larger crystals of biotite, which Avere proper 

 to the rock before the intrusion of the granite. The junction 

 between the two rocks is very sharply defined, and there is no sign 

 of absorption having taken place. (See PI. Ill, fig. 6.) 



"Whin scales. — The marginal zone occurs near the sheepfold at 

 a height of 1200 feet, and is unusually thin ; but there is a well- 

 marked breccia, consisting of angular fragments of Borrowdale 

 rocks in a granitoid matrix. 



Specimen 100 a. — This is a somewhat unusual type, but I 

 have collected it in several other parts of the area, where it always 

 occurs just within the belt of fine-grained acid material that 

 constitutes the extreme edge of the intrusion. It contains large 

 and extremely thin films of biotite which coat the faces of the larger 

 felspar-crystals, and, when seen in transverse section, look like 

 acicular crystals. They are dark grey in colour, the remainder of 

 the rock being pink. 



Under the microscope the rock shows quartz, perthite, orthoclase, 

 albite, and broad (but extremely thin) crystals of biotite which 

 often exhibit the transition into chlorite. Muscovite appears to be 

 absent. There are slight indications of micropegmatitic structure. 



This rock passes gradually into the usual marginal variety, con- 

 sisting of a very fine-grained aggregate of quartz, with a little 

 orthoclase and, in this instance, a few minute flakes of biotite. 

 The actual contact is exposed, and is seen to be perfectly sharp. 



Specimen 101. — This is a specimen of the junction collected a 

 few yards away from the sheepfold. The intrusive portion is a 

 pink felsitic rock, consisting of a fine quartz-orthoclase mosaic with 

 a few minute flakes of biotite. The contact is quite sharp, a large 

 phenocryst of felspar in the Borrowdale rock being cleanly cut 

 across. 



