-6 GEOLOGICAL MEMOIRS. 



useful work of Macculloch. References to the writers and works 

 on the subject which the author has consulted are given, but the 

 names of several good authorities are not mentioned in the list. A 

 series of large folding tables are also appended ; one coloured, which 

 contains a sketch of the position, age, and relation of the crystalline 

 eruptive rocks, with reference to their localities in Germany and 

 adjoining countries ; the others showing the relations of the super- 

 position of the stratified rocks in Germany and other countries, 

 together with their mineral characters and fossil contents. 



In the introduction, which treats of simple minerals as materials 

 for the formation of rocks, the importance of a knowledge of mine- 

 rals for the study of rocks is pointed out, with general instructions 

 as to the method and instruments necessary for the examination of 

 rocks, such as the action of the blowpipe, mechanical division, &c. 

 This is followed by short descriptions of the more important rock- 

 component minerals, arranged according to the classification of Nau- 

 mann. 



The participation of minerals in the formation of rocks is next 

 treated of; (1) as essential characteristics or chief constituents, and 

 (2) as accidental, foreign, or accessory minerals, arranged in the fol- 

 lowing order : — 



A. Minerals which occur as essential components of rocks. 



I. Minerals which of themselves form rocks, 



a. Those which form homogeneous rocks only. 

 Anthracite, stone-coal, brown-coal, ice, rock-salt, gypsum, 



dolomite, marl, brown and red iron-ore, iron-spar, perlite, pitch- 

 stone, obsidian, serpentine, opal, siliceous schist (kiesel-schiefer), 

 hornstone, and fire-stone (flint). 



b. Those which form rocks, not only alone, but also mixed 

 with other minerals. 



Quartz, augite, hornblende, chlorite, talc, clay, and lime- 

 stone. 



II. Minerals which occur as essential components only in com- 

 bination with others. 



a. Those which occur as essential minerals in one variety of 

 rock, but often as accessory in various other rocks. 

 Leucite, nephelin, topaz, zircon, garnet, tourmaline, hyper- 



sthene, smaragdite, and andesine. 



b. Those occurring in some varieties of rock as essential, in 

 others as accessory components only. 



Zeolite (natrolite, scolezite, desmine?), labradorite, ortho- 

 clase, sanidine, albite, oligoclase, iron-chlorite, mica, diallage, 

 and magnetic iron. 



B. Minerals which appear generally as accidental only. 



Titaniferous iron, iron-pyrites, olivine, chabasite, stilbite, chi- 

 astolite, and talc-spar (magnesite). 



