ISLA. GEOLOGY. 247 



surface marked by the frequent protrusion of rugged hills, 

 ridges, and single rocks; in which every possible variety 

 of direction and inclination, from the vertical to the hori- 

 zontal, and from the straight to the curved, may be seen 

 in some place or other ; since however irregular these 

 detached rocks are, they appear all to be portions of 

 one stratified mass. It is every where interfoliated with 

 fine clay slate in different proportions ; the slate predomi- 

 nating in many places to the exclusion of the limestone, 

 while in others again the reverse takes place. But whe- 

 ther the slate be present or absent, it has a schistose frac- 

 ture, although that is generally the result of some almost 

 imperceptible lamina of clay slate by which it is accom- 

 panied. The alternations of the two substances are often 

 so frequent and so delicate, that a specimen cannot be 

 distinguished from clay slate when examined on its sur- 

 face or leading fracture ; while, when viewed on the cross 

 fracture, the limestone alone is seen, and the schist disap- 

 pears. These alternations are generally found at the 

 junction of a considerable mass of the one rock with 

 the other. The limestone is invariably lead blue, although 

 sometimes traversed by white veins, and is commonly of 

 a fine grain ; the clay slate being of the same colour, and 

 often silky and undulated on the surface. 



In addition to the topography of this variety of lime- 

 stone as it is detailed in the map, a few remarks are neces- 

 sary. The principal mass seems to lie in an irregular 

 manner, since it is wide in the middle and becomes con- 

 tracted near the northern shore ; while it also appears 

 to terminate near the farm of Lossit before it reaches 

 the sea. Considering the high angle which the strata 

 occupy, it may be conceived that this part is the extenu- 

 ated edge of the deposit, and that if the strata were 

 here as high above the sea as they are in the interior, 

 they would preserve a corresponding breadth. Though 

 presenting the superficial form of those secondary strata 

 which occupy basin-like cavities among the primary, it 



