NOTES. Clxix 



bable that the unusual sporadic occurrence of certain mineral bodies, and the 

 reasons of their regular or normal specific association, are dependent on the con- 

 currence of many not yet ascertained causes in respect of pressure, temperature, 

 character and degree of fluidity, and rapidity of cooling. Specific differences of 

 association are, however, of great importance in mixed rocks, as well as in dike- 

 or vein-masses ; and in geological descriptions drawn up in the open field, where 

 it has been possible to do so with the described object actually present, care 

 should be taken to distinguish whether any particular associated member is of 

 general prevalence, or at least is rarely absent, or whether its occurrence is only 

 occasional and apparently accidental. The diversity which is found in respect 

 to the elements of a composite rock (ex. gr. trachyte) is also to be observed, 

 as I have already remarked, in the occurrence of the rocks themselves. In 

 both continents there are large territories in which trachytes and basalts, as it 

 were, repel each other, as do basalts and phonolites; and other territories in 

 which trachytes and basalts alternate in considerable proximity to each other. 

 (Compare Jenzsch, Monographic der bb'hmischen Phonolithe, 1856, S. 1 7.) 



( 6:1 ) p. 437. Compare in Bischof, Chemische und physikalische Geologic, 

 Bd. ii. 1851, S. 2288 and S. 2297; and Roth, Monographic des Vesuvs, 1857, 

 S. 305. 



( 612 ) p. 438. Kosmos, Bd. iv. S. 365 (English edition, p. 320). 



( 613 ) p. 438. It is scarcely necessary to remind the reader that the ex- 

 pression " is wanting " only implies that a mineral species has hitherto been 

 looked for in vain throughout a not inconsiderable portion of the volcanoes of an 

 extensive territory. I distinguish between : wanting (i. e. not found) ; rarely 

 found; and frequently present, but yel; not normally and as a characteristic 

 component. 



( 6M ) p. 438. Carl von Oeynhausen, Erkl. der geogn. Karte des Lacher 

 Sees, 1847, S. 38. 



( 615 ) p. 438. See the Bergmannisches Journal von Kohler und Hofmann, 

 5th year, Bd. i. (1792) S. 244, 251, and 265. Basalt, rich in mica, as at the 

 Gamayer Kuppe in the Bohemian Mittelgebirge, is a rarity. I visited this part 

 of the Bohemian mountains in 1792, in company with Carl Freiesleben, who 

 afterwards travelled with me in Switzerland, and who very materially influenced 

 my geognostic and mining education. Bischof doubts mica being ever produced 

 in a pyrogenous manner, and regards it as a product of metamorphic action 

 "par la voie humide;" see his Lehrbuch der chem. und physikal. Geologie, 

 Bd. ii. S. 1426 and 1439. 



( 816 ) p. 438. Jenzsch, Beitrage zur Kenntniss der Phonolithe, in the Zeit- 

 echrift der deutschen geologischen Gesellschaft, Bd. vni. 1856, S. 36. 



