[138]. For the meaning of “talasinga” see p. [55].
[139]. The portion exhibiting the coral structure has a specific gravity of 2·54.
[140]. A good list of references to the early German authorities on the subject is given in the American Journal of Science and Arts for 1831, vol. 20.... Zirkel in his Lehrbuch der Petrographie (1893, vol. i. p. 565) gives most of these and many more recent.... Harker in his paper below named refers to a review of the earlier literature in Verh. naturh. Vereins. Bonn, 1851, vol. 8, and to a more complete bibliography by Meli in Boll. Soc. Geol. Ital. 1881, vol. 9.... British Association Report in 1889 by Professors Rücker and Thorpe on the Magnetic State of the British Isles.... Nature for August and September, 1894, &c.... Harker on magnetic disturbances in the Isle of Skye, Proc. Cambr. Philos. Soc. vol. 10, part 5.... Skinner in Proc. Cambr. Philos. Soc. May, 1894.... Clark in Journ. Roy. Instit. Cornwall, 1890-93.... Folgheraiter in Frammenti concernanti la geofisica, Rome: referred to in Nature, July 27, 1899, and Nov. 8, 1900.
[141]. Nearly all volcanic rocks at all basic are magnetic, owing to the constant presence of magnetite; but magnetic polarity, when the rock-fragment has a negative and a positive pole, is not directly concerned in volcanic rocks with the mineral composition.
[142]. Some of the earliest observations were made on granites and trachytes.
[143]. Geology of the United States Exploring Expedition, 1849, p. 294.
[144]. Quarterly journal Geological Society, vol. li., p. 469.
[145]. Wilkes’ Narrative of the U.S. Exploring Expedition, iii., 185.
[146]. Of the tuffs and clays, almost all submarine and often containing tests of foraminifera and sometimes molluscan shells, about 90 per cent. exhibit simple magnetism in a slight degree, but out of nearly 100 specimens tested none show polarity.
[147]. On p. [357] will be found some notes on the magnetic iron sand that occurs in great abundance in river and stream beds.