350 J. J. Harris Teall — Hornblende-Bearing Rocks. 



mass of the rock ; and (o) in the fact that large crystals sometimes 

 project from the inclusion into the ground-mass. 



The following analyses will give some idea of the variation in the 

 chemical composition of these rocks. 





I. 



II. 



III. 



SiO, 



63-41 



54-09 



52-47 



A1 2 3 



16-92 



15-02 



12-15 



Fe 2 3 



2-67 



4-12 



3-47 



FeO 



2-96 



5-15 



5-23 



CaO 



4-32 



7-72 



9-71 



MgO 



2-08 



7-28 



9-94 



Na 2 



5-18 



1-99 



2-81 



K 2 



2-36 



3-55 



2-26 



H 2 

 CO, 



•- } -64 Ig. ... 



( 1-49 



1-62 

 •54 



100-54 100-39 100-20 



I. Hornblende -porphyrite, intrusive in quartzite, base of Beinn an Fhurain. 

 II. Porphyritic -diorite, base of Beinn an Fburain. 

 III. Plagioclase-pyroxene- hornblende rock, near Incbnadampf, intrusive in limestone. 



Titanic acid, phosphoric acid, and manganese are present in the 

 above rocks, but have not been determined. They have therefore 

 been weighed with SiOa, A1 2 3 , and Fe 2 3 . Specimen I. contains 

 two generations of felspar, and is a typical- porphyritic rock. 

 Specimen II. contains only one generation of felspars. The horn- 

 blende is more abundant than in I., and sometimes occurs in good- 

 sized porphyritic crystals. Specimen III. differs from the others in 

 containing a large amount of colourless pyroxene. 



It is seen to be richer in lime and magnesia, but not richer in iron. 

 In all probability the pyroxene is a nearly pure lime-magnesia 

 bisilicate, and one is tempted therefore to ask whether it may not be 

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

 the dolomitic limestone into which the rock has been intruded. The 

 sporadic manner in which the pyroxene occurs is in favour of the 

 view here suggested. As telling somewhat against this view we have 

 to mention the frequent occurrence of a colourless pyroxene in the 

 more basic plagioclase-hornblende rocks, as, for example, in some of 

 Prof. Bonney's hornblende-picrites x and the allied rocks of War- 

 wickshire described by Mr. Allport as diorites. 2 



One very interesting feature which a comparison of the different 

 varieties of diorite brings out is the variation in the state of crystal- 

 lization in the ground-mass. In the compact rocks the individual 

 felspars are minute, and give more or less lath-shaped sections. It 

 is impossible to determine the precise character of the original 

 ground-mass in consequence of alteration, but it evidently approxi- 

 mated to that of the hornblende andesites. In the medium -grained 

 rocks the microstructure of the ground-mass is thoroughly granitic. 

 Here then we see a gradation exactly similar to that described by 



1 On tbe so-called Diorite of Little Knott, with furtber remarks on tbe occurrence 

 of Picrites in Wales, Q. J. G. S. vol. xli. p. 511. 



2 On tbe Diorite of tbe Warwickshire Coal-field, Q. J. G. S. vol. xxxv. p. 637. 



