346 



absolutely perfect, and, as the crystals are small, they may be well 

 observed in thick sections of the rocks. 



In some rocks the porphyritic constituents have exercised a 

 marked influence on the arrangement of the microlitic groups ; the 

 latter forming a kind of fringe round the former. Certain dykes on 

 the west coast show banded and fluxion structures. 



Mr. ALLPORT has described the intimate relation between pitch- 

 stone and a variety of felsite known as " hornstone." These two 

 rocks occur as modifications of one and the same mass in a vein on 

 the west coast near King's Cove. The red hornstone differs from the 

 pitchstone in being crypto-crystalline and in containing a large amount 

 of colouring matter. It is interesting to note that the red colouring matter 

 (ferrite) makes its appearance as soon as the matrix shows double refrac- 

 tion, that is as soon as crystallization takes place. This agrees with the 

 views of VOGELSANG and ROSENBUSCH that it is squeezed out of the 

 magma during the process of crystallization. 



The felsites of Arran vary considerably in character. Some are 

 compact and without porphyritic constituents ; others contain crystals 

 of bipyramidal quartz, sanidine and plagioclase ; others again are 

 markedly spherulitic ; the spherules being sometimes as large as 

 small peas and so thickly set as to make up the bulk of the rock. 

 The ground-mass of the felsite varies in colour from a bluish black 

 to light grey or yellow. Under the microscope it is crypto- 

 crystalline, micro-crystalline, confusedly crystalline or spherulitic. 

 Sometimes there are traces of granophyric structure. The small 

 spherulitic bodies in the ground-mass of certain felsites are clear and 

 radially fibrous. They give a well-defined black cross. 



Fine examples of porphyritic felsites (liparites) occur at Drumadoon 

 Point, Leac-a-Bhreac and Benan Head. The ground-mass is crypto- 

 or micro-crystalline. The porphyritic constituents are quartz, often 

 bipyramidal, sanidine, plagioclase. Microlites of hornblende occur in 

 some varieties. As the porphyritic monoclinic felspar is usually clear 

 and glassy these rocks would be liparites according to the nomenclature 

 we have adopted. ZIRKEL calls attention to their close relation to the 

 trachytes. The " claystones " of the earlier varieties occur abundantly 

 as dykes They are less markedly porphyritic than the rock above 

 referred to. Porphyritic grains of quartz frequently occur and they are 

 usually surrounded by a zone which extinguishes simultaneously with the 

 central grain. This is the " quartz globulaire " of French authors. 



On the Corriegills shore there occurs a remarkable variety of spheru- 

 litic felsite, the microscopic structure of which has been described by Mr. 

 ALLPORT (i) and Professor BONNEY. < 2 > The spherules are often as large as 

 small peas (see Fig. 1, Plate XXXIX.). In thin section they show a radially 

 fibrous, and frequently also a concentric structure, due to the distri- 

 bution of brown granular matter. They do not show a well-defined 



(1) G.M., 1872, p. 540. 



(2) G-.M., 1877, p. 499. 



