206 Physical Notices on the Bay of Naples. 



II. Volcanic Conglomerate or Breccia. A rock composed 

 of the various simple volcanic rocks in the state of fragments 

 and united by a basis. It is occasionally agreeable in its ap- 

 pearance. 



III. Tufa. This substance is best known as the work of 

 extinct volcanos, but is a frequent production of Vesuvius 

 even at the present day. It may be considered as a hardened 

 mud of the same description as buried Herculaneum. I have 

 a piece produced by the eruption of 1822, which has taken 

 the complete impression of the leaf of a tree. I propose to 

 resume the subject of tufa in two future Numbers of these 

 " notices ;" on the phenomena of the buried cities, and on the 

 tufas of the Bay of Naples. = 



IV. Volcanic ashes, or more properly sand, is one of the 

 most abundant productions of Vesuvius. A shower of it ge- 

 nerally precedes great eruptions ; and from the subtile division 

 of its parts it is capable of being sustained long in the air, and 

 carried even to Egypt and Syria in a. d. 79, to Constanti- 

 nople in 472 and 1631, and all over Calabria in 1139 and 

 J 794. In 79 it buried Pompei to the depth of many yards ; 

 and in 1822 the excavated parts of that wonderful city were 

 again covered with the sand to the depth of several feet, and 

 it lay finger deep in Naples. When chemically examined it 

 was found (what is very remarkable) to contain a minute 

 quantity of gold. The last portion that fell, such as I gather- 

 ed it in the Atrio del Cavallo, is an almost impalpable powder. 



From the extremely minute quantities of Obsidian or volcanic 

 glass and Pumice afforded by this volcano, I prefer consider- 

 ing them as simple minerals rather than rocks. On the for- 

 mer class of substances we must now pass a few hasty remarks, 

 which must be confined within very small compass. 



Following, then, the system of Mohs, and stopping only at 



203, under Severus. 472, 512, 685, 993, * 1036. Probably the first time 

 that lava was ejected. 1049, 1138, 1139, 1306, 1500. (^Monte Nuovo ap- 

 peared in 1538,) * J 631, 1660, 1682, * 1694, 1701, 1704, 1712, 1717. La- 

 va continued to flow for eleven years, 1730, 1751, 1754, 1759, 1765, 1766, 

 *1769, 1776, 1777, 1778, * 1794, 1802, 1803, 1804, 1806, 1809, 1810, 

 1812, 1816, 1817, 1818, 1819, 1820, * 1822, February and October. 1828, 

 March 22. Forty-five eruptions have been above enumerated, and some- 

 times for years together Vesuvius has continued slight eruptions. 



