558 Bcriens — A. Harher — Roclni of Shye. 



by the invading granite majiraa with the consequent production of 

 hybrid rocks of abnormal chemical and mineral composition, and 

 the detailed accounts of the peculiar composite sills and dykes 

 outside the mountain tract, form perhaps the most important and 

 interesting feature of the memoir. In the case of Skye the author 

 finds that the heterogeneity of these intrusions is invariably due, not 

 to differentiation subsequent to intrusion, but to successive injections 

 of different rock-magmas, the acid magma showing a tendency to be 

 guided by a pre-existing basic dyke or sill. 



Of the third and final phase of igneous activity in Skye which 

 was characterised by minor intrusions in the form of sills and 

 dykes, the most important role was played by the great group 

 of basic sills previously referred to, which are intercalated among 

 the basaltic lavas to such an extent that they and not the basalts 

 really form the salient features of the plateau country. Amongst 

 a minor group of basic sills an interesting type is the peculiar 

 composite sills of Roineval, to one of the constituents of wliich the 

 author gives the provisional name of Mugearite. This and Marscoite 

 (a hybrid of gabbro and granite, produced, however, before intrusion) 

 it is a relief to see are the only new rock-names whicli appear in 

 a memoir which deals with so man}'- I'ocks of exceptional composition. 

 To the rocks composing most of the basic dykes and sills we are 

 glad to find that Mr. Harker applies the name olivine-dolerite, 

 although he has not been able to avoid altogether the use of his 

 term diabase for some of the coarser-textured types near to gabbro. 



In an interesting penultimate chapter is given a general review 

 of Tertiary igneous activity in Skye. As the cause of this activity 

 two distinct elements of crustal strain are recognised, the one 

 regional and probably related to the great Atlantic depression, and 

 the other local : to the first are attributed the fissure eruptions of 

 basalt, the great group of basic sills, and the system of parallel 

 dykes, and to the second the central volcanic eruptions, the plutonic 

 intrusions, and the radial group of dykes peculiar to the Cuillin 

 district. 



In the last chapter, the physical features and scenery of the 

 island and their relation to the geology are discussed. A very 

 complete bibliography of the geological literature referring to the 

 igneous rocks of Skye is given in an Appendix. 



The memoir is illustrated by 84 text-figures and 27 plates„ 

 References to the figures in the text might have been with advantage 

 more often accompanied by indications of the pages on which 

 they occur. 



In conclusion, it remains to congratulate Mr. Harker on the 

 completion of an arduous but very successful piece of work, and 

 the Geological Survey on the production of a volume which with 

 its numerous and excellent illustrations can be offered to the public 

 at so low a price. 



