INDUSTRIAL PROGRESS DURING THE YEAR 1874. lxxiii 



deira, the Canaries, the Cape Verde Islands, Ascension, St. 

 Helena, and Tristan d'Acunha belong; all of which, as the Chal- 

 lenger's late soundings have shown, formed parts of a mount- 

 ain range, comparable in extent, elevation, and volcanic char- 

 acter with the Andes of South America. The area of these 

 Tertiary eruptive rocks in Ireland and the Hebrides is esti- 

 mated at over 2200 square miles, and their aggregate thick- 

 ness from about 1300 feet in Antrim to between 3000 and 

 4000 feet in the island of Mull. These eruptions were sub- 

 aerial, and belong to three periods the first probably of later 

 Eocene, the second and third of Miocene age. Zirkel recog- 

 nizes among the tertiary volcanic rocks of Ireland trachytes, 

 felsites, and pitchstones, as well as dolerites. The last, 

 whether coarsely crystalline or basaltic, have essentially the 

 same mineral composition, a fine-grained base or glass from 

 which the feldspar, augite, and chrysolite have individually 

 crystallized. The basalts are in large part of augite, and 

 are often highly charged witli titanic iron ore. Lava streams, 

 dikes, mountain masses, scoria3, ashes, and volcanic brec- 

 cias, inclosing fragments of older sedimentary rocks, are met 

 with, and the currents of the basic lavas have been traced 

 fifty or sixty miles, though the trachytic ones appear to have 

 flowed much less distances. In these tertiary eruptions, as 

 in those of Paleozoic age, above noticed, the trachytic seem 

 to have preceded the augitic lavas, and the close of the pe- 

 riod in the later miocene was marked by small volcanic cones, 

 as in Auvergne. In each of these periods there were out- 

 flows, with intervals of repose sufficiently long to allow of 

 the sub-aerial decay of the surfaces, giving rise to beds of clay 

 and to aluminous iron ores. The so-called bole or laterite 

 is the doleritic rock decayed in sitil, and retaining its orig- 

 inal structure. 



The sub-aerial decay of crystalline rocks, noticed in our 

 Recovd last year, has been further studied by Hunt. He has 

 shown that at the western base of the Hoosac Mountain, along 

 the line of the great tunnel, there are from 200 to 300 feet in 

 thickness of gneiss, so soft as to be excavated like clay and 

 gravel, yet retaining the structure and arrangement of the 

 original beds, which are nearly vertical. He has further de- 

 scribed the same phenomenon at the northwest base of the 

 South Mountain in Pennsylvania, where nearly vertical soft 



