GEOLOGY.—LECTURE II.

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micaceous schist. Porphyry is seldom found; there isno serpentine in this district, and very little sienite,except at the bottom of the northern mountains. Somequarries of primitive marble occur at the eastern foot ofthe granite mountains of Dublin.

Amongst the Scotch mountains, Ben Nevis , thehighest in the island, is nearly 4,500 feet above thelevel of the sea; it is composed of granite, micaceousschist, and gneiss, and most of the other great moun-tains are similarly constituted. Porphyry, sienite, andmarble occur, forming as it were smaller chains uponthem, and the serpentine is principally limited to a spoton the W'est coast near Portsoy, and a spot on the coastof Ayr.

Identical or very similar relations exist in the moreextensive mountain chains belonging to great conti-nents, (as may be learned from the writings of de Saus-sure, de Luc, Pallas, Born, Werner, Humboldt, andRaymond), in the Swiss Alps , the Pyrenees, the Saxonrocks, and the mountains of America ; and from theconformity of most of the later observations, there seemsgreat reason to believe that the more minutely inquiriesare carried on, the more the earth is explored, thegreater uniformity and distinctness will be observed inthe relationship of the positions of rocks, and in theirgeneral arrangement.

The chains of primary rocks, often extending formany hundred miles, in their geographical position, areof considerable importance in preserving the harmonyof the great series of natural events.

The heat of the atmosphere is derived from the agencyof the rays of the sun on the surface of the earth, and itdiminishes in proportion to the height; so that in allseasons, and in all climates, at two or three miles high,