203 Mr. Bkyce on the Geology of the Island of Bate. 



instances which require special mention, as presenting phenomena some- 

 what unusual. 



Between Ascog and Kerrycroy, a greenstone dike, five yards wide, 

 which has run parallel to the shore for some distance, gradually retires 

 from it, toward the latter place, and striking the inland cliff already 

 mentioned, whose direction here coincides with that of the dike, it forms 

 the perpendicular face of the cliff in front of the sandstone, rising like a 

 wall to the height of 20 or 30 feet. The direction of the cliff soon changes, 

 however, and the dike then enters the hill behind, and is lost. By this 

 fortunate coincidence of the two directions, the largest surface that I 

 have ever seen exposed in the case of a dike is completely laid bare, and 

 thus the structure is revealed in the most satisfactory manner. These 

 dikes, as is well known, are usually prismatic across, not vertically, as 

 the overlying trap. The reason is obvious enough ; the sides of the dike 

 acted as the cooling surfaces to the fused and liquid mass within ; the 

 imprisoned caloric of course passed off in a direction perpendicular to these 

 surfaces, and hence the divisional planes are also perpendicular to the 

 sides — or the dike is prismatic across. The following sketch will help to 

 convey some idea of this interesting dike ; the prisms are mostly pen- 

 tagons and hexagons. 



No. 7. 



Side view of whin dike between Ascog and Kerrycroy. 



The other dike, which deserves special notice, traverses the Kilchattan 

 limestone. Its direction is very nearly that of the dip, and the effects 

 are well seen at the eastern side of the quarry. Along the plane of con- 

 tact the limestone is altered to the state of a granular saccharine marble, 

 which, on the application of a slight pressure, crumbles into a fine powder. 

 This is succeeded by a hard crystalline marble, the crystals appearing in 

 distinct flakes. Between this and the last change, which is one of simple 

 induration, there are many gradations. Similar effects are common at 

 the contact of limestone with plutonic rocks ; in some localities they are 

 accompanied by other singular changes of a chemical nature. Magnesia, 

 and sometimes silica and alumina, are introduced into the composition of 



