Vol. 50.] 



ACID ROCKS OF THE INKER HEBRIDES. 



225 



Sometimes the laminae of flow have been disrupted, and broken 

 portions of them have been carried onward and enveloped in the 

 still unconsolidated material. Some portions of the dyke are 

 richly spherulitic, the spherulites varying from the size of small 

 peas up to that of tennis-balls. Occasionally two large spherulites 

 have coalesced into an 8-shaped concretion. In its flow the acid 

 rock has caught up and enclosed portions of the gabbro walls. In 

 fig. 2 one such block is shown with the lines of flow sweeping 

 round it. Another portion of gabbro at the right hand of the figure 

 may be a projection of the wall, but is more probably a second 

 enclosed block. I did not, however, trace the felsite completely round 

 its northern end. 



Fig. 2. — Plan of granojpihyric dyJce cutting and enclosing gabbro, 

 Druim an Eidhne, Skye. 



I have had thin slices cut from the gabbro-block enclosed within 

 this dyke, and have submitted them to microscopic examination by 

 Mr. Teall, who has been so good as to furnish me with the following 

 memorandum regarding them : — " The rock is medium-grained, dark 

 green, and massive. Its principal constituent is a turbid plagio- 

 clase, which sometimes shows a tendency to assume rectangular 

 forms. The ferro-magnesian constituents occur in greenish or 

 yellowish-brown patches, which sometimes have the form of the 

 augite in the ophitic gabbros and sometimes have ill-defined bound- 

 aries, owing to the fact that the substances of which they are com- 

 posed penetrate the surrounding plagioclase. These patches consist 

 partly of uralitic hornblende and partly of an extremely fine 

 aggregate into which mica probably enters as a constituent. Quartz, 

 magnetite, and apatite also occur in the rock, which is evidently an 

 altered gabbro." 



The alteration which the material of this block has undergone 

 is such as might naturally be expected to occur in a lump of 

 gabbro enclosed within a mass of eruptive rock. But as it might 



