Relations of the Secondare/ Strata in the Isle of Arran. 27 



bands separated from each other by courses of shale called "till" by the 

 natives, with many fine specimens of Productee. 



All the beds described in this coal series, viz. from the red to the black 

 limestone, rise high on the sides of ihe schist mountains, which occupy the 

 two terraces of Laggan on their inland or south-western flank. The two dis- 

 tant beds of limestone having been traced to a considerable elevation in the 

 sides of the mountain, preserving the same range and dip as upon the shore, 

 there can be no doubt that the intervening carboniferous and sandstone beds 

 are also prolonged up the face of the mountain : we calculated the distance 

 between these two beds of limestone to be from twelve to fourteen hundred 

 paces. The account therefore which has represented this coal-field as a small 

 triangular space inclosed by limestone must be erroneous. The error probably 

 originated in the fact that the coal works here were interrupted by two trap 

 dykes, which in intersecting each other, cut off a very small triangular portion 

 of the coal-field, the only part of it in which the works were prosecuted*. 



In a descending order, the next important beds are strong white grits, which, 

 near Millstone Point, are much dislocated, and piled upon each other in large 

 blocks of prismatic form : from thence, turning into Laggan-twine Bay, the 

 strata consist of alternations of white sandstone and shale, the latter beinjj 

 generally of a red colour, and somewhat more indurated than nearer the coal 

 pits. The open bay which follows is filled with small boulders, and on the 

 other side of it is a repetition of the sandstone series with the same dip N.N. W.; 

 but the angle is increased to 43'^ and 50°. A series of beds of trap next suc- 

 ceeds, which may be generally described as follows : 



1 . Dark porphyritic trap, with an earthy base, containing crystals of diallage 

 metalloide. 



2. Black porphyritic greenstone. 



3. Lighter coloured, and more earthy varieties of porphyritic greenstone. 



4. Claystone, with veins of green earth. 



5. Various amygdaloids abounding in crystals of carbonate of lime, and 

 having some of the cells filled with finely crystallized green earth ; others with 

 a dark steatitic substance, and more rarely with zeolite f. 



These trap rocks occupy the shore for about a quarter of a mile, during which 

 space they preserve all the appearance of regular beds, having the same range 

 and dip as the superior and inferior strata of the coast section. They are fol- 

 lowed by beds of shale so indurated as to put on all the characters of Lydian 



* See the work of Mr. Iltadrick on the Island of Arran, quoted above. 



t This description is taken from notes made on the spot. The specimens we collected from 

 this remarkable group for the purpose of more careful examination were unfortunately lost. 



e2 



