410 RECENT WOEK OE THE GEOLOGICAL SURVET 



in the limestones. He has called attention to the presence of augite 

 in addition to the hornblende, and suggests that the pyroxene may- 

 be due to the absorption by the igneous magma of a certain amount 

 of the dolomite-limestone into which the rock has been intruded. 



These interesting observations of Mr. Teall are confirmed by the 

 results of the detailed examination of the Assynt region. Indeed, 

 during the season of 1885, when the limestone-plateau of Inch- 

 nadamff was mapped, it became sufficiently obvious that, with few 

 exceptions, the intrusive rocks in the limestones are more basic than 

 those in the quartzites. It was also noted that the development of 

 pyroxenes is a characteristic, not only of the thinner bands in the 

 calcareous series, but of those portions of the great sheet of igneous 

 material east of Ledbeg which are in immediate contact with the 

 marble. 



The following varieties are met with in the lower and upper 

 zones of quartzite : — (a) compact, fine-grained, pink or grey felsite, 

 with or without porphyritic quartz; (b) porphyritic felsite, with 

 crystals of felspar and hornblende set in a f elsitic ground-mass ; (c) 

 the former shades into a highly crystalline rock in which large 

 crystals of orthoclase and albite, with beautiful zonal banding, occur 

 in a micro-crystalline ground-mass consisting of felspar and horn- 

 blende ; (d) porphyritic diorite in which hornblende-crystals are 

 porphyritically developed in a crystalline matrix of plagioclase 

 felspar with some hornblende. 



The igneous sheets and dykes in the Ghrudaidh limestones 

 (Group I.) consist mainly of diorite, some of them being fine- and 

 others coarse-grained. With the hornblende and plagioclase felspar 

 augite is occasionally associated. The dyke occurring in the Eilean- 

 Dubh limestone is usually a grey felsite or hornblendic felsite. 



The macroscopic characters of the great intrusive mass, extending 

 from Ledbeg eastwards by Loch Borrolan to a point near the Loch- 

 Ailsh road, are somewhat different from the foregoing types. The 

 greater portion is highly granitoid, consisting mainly of crystals of 

 felspar (albite, microcline, orthoclase) with secondary quartz. In 

 those localities where hornblende is present, the rock resembles a 

 hornblendic granite. Where the intrusive mass has come in contact 

 with the limestone, pyroxene has been developed ; indeed the molten 

 material must have absorbed a large quantity of calcareous matter, 

 for in many places it effervesces freely. 



5. Contact-Metamorjphism. 



Where the bands are comparatively thin, not much alteration is 

 observable in either the quartzites or the limestones, except a slight 

 induration of the strata along the edges of the intrusive masses. 

 But where they reach a considerable thickness, some remarkable 

 changes occur. For example, on the slopes of Ben Garbh, south of 

 Loch Assynt, the quartzites, traversed by a massive sheet of por- 

 phyritic felsite, have been so altered that the two rocks are welded 

 together. In the case of the great granitoid mass at Ledbeg and 



