39S Scientific Intelligence. — Geology. 



mens of fossil insects, found in the oolite limestone of Solenhof. 

 The following are mentioned by Wagner. Thirty species, many 

 undescribed, of the class Crustacea ; several species of the genera 

 ^schna, Agrion, Myrmeleon ; so that, at the time of the de- 

 position of this limestone, thei*e lived at least three genera of 

 Hymenopterous insects. A fossil Sirix, in the collection, may 

 be considered as the representative at that period of the order 

 Neuroptera. Von Schlotheim mentions an impression in this 

 limestone of an insect, which he conjectures to be nearly allied 

 to the moth named Sphinx ligustri ; and also a fossil beetle, 

 allied to the genus Cerambyx. The Spider class appears to 

 have existed also at this time, as is shewn by a fossil nearly al- 

 lied to the Solpuga Fabr Galeodes of Olivier. 



12. Antique Green Porphyry. — In the Island of ^Egina, one 

 of the Greek Islands, Captain Boblaye, a French engineer, dis- 

 covered rocks of antique green porphyry (ophite), which he 

 refers to the porphyries of the coal formation. 



13. Durability of Stones. — When the felspar of the granite 

 rocks contains little alkali, or calcareous earth, it is a very per- 

 manent stone ; but when in granite, porphyry, or syenite, either 

 the felspar contains much alkaline matter, or the mica, schorl, or 

 hornblende, much protoxide of iron, the action of water contain- 

 ing oxygen and carbonic acid on the ferruginous elements tends 

 to produce the disintegration of the stone. The red granite, 

 black syenite, and red porphyry of Egypt, which are seen at 

 Rome in obelisks, columns, and sarcophagi, are amongst the 

 most durable compound stones; but the grey granites of Cor- 

 sica and Elba are extremely liable to undergo alteration : the 

 felspar contains much alkaline matter, and the mica and schorl 

 much protoxide of iron. A remarkable instance of the decay 

 of granite may be seen in the hanging tower of Pisa ; whilst the 

 marble pillars in the basement remain scarcely altered, the gra- 

 nite ones have lost a considerable portion of their surface, which 

 falls off continually in scales, and exhibits everywhere stains from 

 tVie formation of peroxide of iron. The kaolin, or clay, used in 

 most countries for the manufacture of fine porcelain or china, 

 is generally produced from the felspar of decomposing granite, 

 in which the cause of decay is the dissolution and separation of 

 the alkaline ingredients. Water is capable of dissolving, in 



