﻿464 PROF. E. W. SKEA.TS ON THE GNEISSES [Aug. 1 9 TO,. 



recently had access, so that my observations were completed before 

 I could read some of the papers dealing with this subject. Prof. Judd, 1 

 in dealing with the intrusion of igneous masses into the andesites of 

 Mull, describes the alteration of pyroxenes, hornblendes, aud micas 

 into finely granular aggregates of an almost colourless pyroxene 

 and magnetite grains. Secondary biotite also occurs, and the 

 crystals increase in size as the intrusive mass is approached. 



The description by Dr. Marr & Mr. Harker 2 of the intrusion of 

 the Shap Granite into the Borrowdale andesites indicates that some 

 of the effects produced are also noticed in the rocks which I have 

 examined. 



A later memoir by Mr. Harker 3 deals with rocks of more basic 

 composition, and describes the alteration of basic lavas by the 

 intrusion of gabbros and granites. 



Mr. A. L. Hall 4 describes the intrusion of granite into felsite 

 north-east of Pretoria; he shows that the contact is sharp, that 

 granite-veins penetrate the felsite for a few feet, and that no veiy 

 obvious change in the felsite is observed in the field. 



Two papers by Mr. Kynaston appear to provide the closest 

 parallel to the evidence which I have obtained. In a Survey 

 memoir by several authors, Mr. Kynaston 5 describes the intru- 

 sion of the Ben Cruachan granite into masses of porphyrite. This 

 description will be referred to again, in connexion with the origin 

 of the gneissic rocks described above. The effects due to contact- 

 metamorphism include the alteration of the augite and hornblende 

 of the andesites. The augite passes into uralitic hornblende or 

 flakes of pale-green hornblende, but little secondary biotite is 

 formed. In the more altered rocks the ground-mass is completely 

 reconstructed, and consists of a granular mosaic of felspar and 

 quartz with secondary biotite and green hornblende. The felspar 

 phenocrysts are also frequently honeycombed with inclusions of the 

 ground-mass. 



Another and earlier paper of Mr. Kynaston's 6 describes the- 

 alteration of the andesites by the intrusion of granite in the 

 Cheviot Hills. The effects described are in many respects similar 

 to those that I have noted above, but certain interesting points of 

 difference arise. Mr. Kynaston shows that silica and potash are more 

 abundant in the granite, and soda in the andesites. Intermediate 

 dykes (lamprophyres) and acid dykes penetrate the andesites and 

 represent ' complementary ' dykes. Contact-metamorphism in the- 

 andesites extends for half a mile from the contact. The normal rock 

 is an enslatite-andesite. In the less altered rocks are produced 

 minute granules of secondary pyroxene, ragged minute secondary 



1 Quart. Journ. Geol. Soc vol. xlvi (1890) p. 370. 



2 Ibid. vol. xlvii (1891) p. 266. 



3 * The Tertiary Igneous Rocks of Skye ' Mem. Geol. Surv. 1904, p. 50. 



4 Rep. Trausvaal Geol. Surv. for 1904 (1905) p. 40. 



5 ' Geology of the Country near Oban & Dalmally ' (Expl. of Sheet 45) Menu 

 Geol. Surv. Scot. 1908, p. 96. 



G Trans. Edin. Geol. Soc. vol. vii (1899) pp. 390-415. 



