356 PROF. J. ^^. judd on the propylites 



obtained in tlie deep Oorry of Tomsleibe, on the north-western side of 

 the mountain, are found to be a true diorite; and a series of specimens- 

 can easily be obtained exhibiting every gradation from the one type 

 to the other. Some of the intermediate types exhibit the various 

 kinds of " granophyric " structure in a very beautiful manner. The 

 composition of this dioritic mass, which, in its upward development, 

 gradates into a hornblende-andesite, is illustrated by the analysis 

 given at p. 349. 



3. '"'■ Lacoolites:' — ^lu 1874 I described acid, igneous rocks 

 as being intruded among the strata of the Western Isles, and 

 stated that they tend to form thick, lenticular masses, which are 

 generally confined to within moderate distances from the great 

 centres of eruption *. Mr. Gilbert has since proposed to call 

 intrusive masses of this type by the name of Laccolites f. Like the 

 " cupolas " they are much more highly crystalline in their central 

 than in their peripheral portions ; the outside of such masses may 

 be a true andesite, while the central portions exhibit the holocrys- 

 talline or granitic structure of a diorite. Where the country has 

 undergone much denudation, it may often be impossible to state if a 

 particular mass should be referred to the class of " Quellkuppen" or 

 of " Laccolites," but that both types occur in the Western Isles of 

 Scotland, there cannot be any doubt. 



YI. XaTURE of the ORiaiNAL EOCKS FROM WHICH 



THE Propylites were formed. 



In endeavouring to determine the exact nature of the rocks grouped 

 under the general name of " felstones," in the Western Isles of 

 Scotland, very great difficulty is experienced owing to the excessively 

 altered state both of the " phenocrysts " (to use Iddings's usefiil 

 term) and of the ground-mass in which they are imbedded. Deter- 

 minations of specific gravity and the partial or complete analysis of 

 the rocks are, of course, of much value in deciding the place of 

 the several varieties in a classificatory system, yet it is chiefly upon 

 other methods that I have been led to rely in making a study of 

 these very obscure rocks. The alteration which has gone on in 

 them, though often extreme, is not unfrequently found to be more 

 more or less local in character ; and, in the same mass, portions 

 exhibiting very different stages of the change may often be found. 

 It has been my object, in the repeated visits I have paid to the 

 district during the last fifteen years, to trace the much altered and 

 obscure rocks to points where their phenocrysts and ground-mass can 

 be studied in a less altered form ; and although in some instances it 

 has been long before I was able to resolve all the difficulties that 

 have presented themselves, yet in the great majority of cases this 

 method has led to more or less satisfactory results. 



Fortunately, too, several very able investigators have been en- 

 gaged, during the same period, in the investigation of the very 



* Quart. Journ. Geol. Soc. toL xxs. (1874), p. 268. 



t Geology of the Henry Mountains, U.S. Geol. Survey, 1877. 



