540 



S. AUjoort — On the Igneous Mocks of Arran. 



Water 1 



Silica 



Alumina 



Ferric Oxide 



Ferrous Oxide 



Manganous Oxide' 



Lime ..,, 



Magnesia 



Potassa 



1. 



8-23 

 73-90 

 10-12 



1. 



8-14 

 73-78 

 10-08 



trace 

 1-23 1-25 



1-19 1-15 



trace 

 2-42 2-44 



2-64 2-66 



99-73 



II. 



1. 



3-58 



78-05 



11-12 



t 



1-08 

 0-15 

 0-98 



trace 



3. 



3-59 

 77-92 

 11-21 



1-17 

 0-17 

 0-88 



trace 

 trace 



4-94 



4-92 



99-50 99-90 99-86 100-80 100-61 



III. 





1. 



2. 



1-47 



1-47 



78-32 



78-02 



11-39 



11-46 



1-67 



1-61 



0-13 



0-13 



trace 



0-20 



0-20^ 



7-62 



7-72 



Felsites. — On tlie east and west coasts of the southern half of the 

 island there are several dykes of a light grey felsite, which cut 

 through the Carboniferous sandstones in precisely the same way as 

 the pitchstones and basalts ; and sometimes even form portions of 

 the same dykes. These are the rocks frequently mentioned by Dr. 

 Bryce as claystones ; they appear to have been but very imperfectly 

 understood, and as they are far more common than the pitchstones, 

 it is singular that Zirkel should have given no description of them 

 in his account of the Arran rocks. 



These felsites vary considerably in appearance, some being dis- 

 tinctly spherolitic, while others can scarcely be distinguished from 

 the so-called quartz porphyries ; it will, in fact, shortly appear that 

 they possess an intermediate character between the latter and the 

 pitchstones. Eeferring to the preceding account of their general 

 microscopic structure, it will be convenient to describe, in the first 

 place, a rock of very different appearance, but of essentially the 

 same structure and composition. 



Near the small pitchstone vein on the Corriegills shore, there is 

 an intrusive vein of a very remarkable character. 



Glohular Felsite. — The rock was described by MacCuUoch, and 

 has been frequently mentioned by subsequent writers as globular 

 pitchstone, perlitic pitchstone, claystone, and hornstone. It is not 

 mentioned by Zirkel in the paper previously quoted. Dr. Bryce 

 observes : " Mineralogists have long regarded this curious rock with 

 much interest, and various opinions have been held respecting its 

 true relations, some considering it as allied to claystone, and others 

 to pitchstone."^ It will soon be evident that a microscopic examina- 

 tion of one or two thin sections is quite sufficient to establish its 

 true character. 



It is a hard, tough rock of dull greenish-grey colour, full of 



1 In No. I. 2-37 was lost in water bath. 



„ II. 1-32 do. do. 



„ III. 0-65 do. do. 



2 The whole of the iron in No. I. was found "to be in the state of ferrous oxide, 

 and that fact is sufficient evidence that the manganese exists as manganous oxide." It 

 is worthy of remark, that the felsitic portion of the rock contains more silica than 

 the pitchstone, a result quite in accordance with that obtained by microscopic ex- 

 amination. 



^ Geol. of Arran, p. 77. 



