with sets of concentric circles, consisting of line black lines most mathematically drawn, with their intervals often filled up by portions of parallel circular segments, an appearance similar to that produced on the surface of some of the agates imbedded in trap, and which on that account are called ocular. These black lines are perfectly superficial.
The land about this harbour is remarkably disposed in small elevated hillocks, producing an irregularity of aspect similar to that of the Oban shore. The mode adopted in making the canal, by cutting away the edges of these hills, so as to form a continuous embankment against them for a considerable space, has brought their structure to light, and given great facility to the investigation of their composition. They are formed of a continued alternation of beds, elevated to an angle of 80° or upwards, and in a general view appearing to be nearly vertical. On examining these beds, they are perceived to consist of the following substances.
- Coarse graywacke, very well characterized, and precisely corresponding to the definition, inasmuch as it consists of grains of quartz cemented by clay slate.
- The same rock, in which grains of felspar as well as of quartz are cemented by clay slate.
- Coarse grained graywacke of a slaty fracture, or graywacke slate.
- A similar rock of a much finer texture.
- A perfectly homogeneous clay slate, not to be distinguished from the finest varieties of the primitive sort.
- A similar slate of a pale greenish gray colour, and silky lustre, approaching in character to chlorite slate.
Vol. II.3 k