\ 



ROCKS. 251 



ino" its recent formation, bears a resemblance to Jura lime- 

 stone, has been recognized as a product of the sea and of tem- 

 pests.* 



Composite rocks are definite associations of certain oryctog-- 

 uustic, simple mmerals, as feldspar, mica, solid silex, ausrite, 

 and nepheline. Rocks very similar to these, consisting of the 

 same elements, but grouped differently, are still formed by 

 volcanic processes, as in the earlier periods of the world. The 

 character of rocks, as we have already remarked, is so inde- 

 pendent of geographical relations of space,! that the geologist 

 recognizes with surprise, alike to the north or the south of 

 the equator, in the remotest and most dissimilar zones, the 

 familiar aspect, and the repetition of even the most minute 

 characteristics in the periodic stratification of the silurian 

 strata, and in the effects of contact with augitic masses o^' 

 eruption. 



We will now enter more fully into the consideration of the 

 four modes in which rocks are formed — the four phases ot" 

 their formative processes manifested in the stratified and uu- 

 stratified portions of the earth's surface ; thus, in the endog- 

 enous or erupted rocks, designated by modern geognosists as 

 compact and abnormal rocks, we may enumerate the follow- 

 ing principal groups as immediate products of terrestrial ac- 

 tivity : 



1 . Granite and syenite of very different respective ages ; 

 the granite is frequently the more recent,| traversing the sy- 

 enite in veins, and being, in that case, the active upheaving 

 agent. " Where the granite occurs in large, insulated masses 

 of a faintly-arched, ellipsoidal form, it- is covered by a crust or 

 shell cleft into blocks, instances of which are met with alike 

 in the Hartz district, in Mysore, and in Lower Peru. This 

 sea of rocks probably owes its origin to a contraction of the 

 surface of the granite, owing to the great expansion that ac- 

 companied its first upheaval. "§ 



Both in Northern Asia, II on the charming and romantic 

 shores of the Lake of Kolivan, on the northwest declivity of 



* Near Teguiza. Leop. von Buch, Canarische Inseln, s. 301. 



t Leop. von Buch, op. cit., p. 9. 



t Bernhard Cotta, Geognosie, 1839, 8. 273. 



§ Leop. von Buch, Ueber Granit und Gneiss, in the Abhandl. der Berl. 

 Akad. for the year 1842, s. 60. 



II In the projecting mural masses of granite of Lake Kolivan, divided 

 into narrow parallel beds, there are numerous crystals of feldspar and 

 albite, and a few of titanium (Humboldt, Asie Centrale, t. i., p. 29.0, 

 Qustav Rose. Reisc nncli dcm Ural, bd. i., s. 524). 



