the Earth's Contraction from Cooling. 365 



clase ; of which felspars the first two are lowest in silica, 45-55 

 per cent., the third intermediate in amount of silica, or 60 to 64 

 per cent., the last two the highest, 65 to 75 per cent. ; (4) mag- 

 netite (or magnetic iron-ore, Fe 3 O 4 , but often titaniferous) ; and 

 (5) perhaps quartz or free silica, and mica. Serpentine and 

 chlorite are omitted from the list on the ground that they are 

 always metaniorphic. The question with regard to quartz is 

 discussed below. 



(b) The principal rocks in their relation to the subject before 

 us fall naturally, as Elie de Beaumont first formally announced*, 

 into two series, although there are intermediate kinds through 

 which the two graduate into one another : — one, low in silica, or 

 basic, containing less than 56 percent, of silica; the other, high 

 in silica, or acidic, containing 56 per cent, or more. 



The basic rocks include dolerite and the related igneous rocks, 

 made up of augite and Labradorite, with sometimes anorthite or 

 oligoclase, often chrysolite, and generally magnetite in dissemi- 

 nated grains, and varying in specific gravity mostly between 2*8 

 and 32. 



The acidic comprise (1) most trachyte and related felspathic 

 rocks, consisting of one or more of the felspars oligoclase, albite, 

 or orthoclase, with usually a little hornblende and magnetite, 

 and sometimes mica, and not unfrequently free quartz — sp. gr. 

 = 2'5-2*75; (2) syenite, consisting of orthoclase and horn- 

 blende with quartz — sp. gr. = 2*9-3*1 • hyposyenite, consisting 

 of orthoclase and hornblende without quartz — sp. gr. = 2*9- 

 3*2 f; diorite, consisting chiefly of oligoclase or albite with 

 hornblende — sp. gr. =2*80-3'l* granite, consisting of ortho- 

 clase (sometimes along with albite or oligoclase), quartz, and 

 mica — sp. gr. =2 , 6-2*75. 



(c) These igneous rocks are also conveniently arranged with 

 reference to their origin into an iron-bearing and a compara- 

 tively iron-free series. 



The former include the rocks containing as essential consti- 



* Bull. Soc. Geol. de Paris, (II.) vol. iv. p. 1253 (184/). De la Beche 

 has the idea in chapter xviii., on Igneous Rocks, of his ' Geological Re- 

 searches ' (1834). Bunsen uses it in his memoir on the Volcanic Rocks 

 of Iceland (Pogg. Ann. vol. lxxxiii. pp. 201 (1851); and also Durocher 

 later, in his memoir on Comparative Petrology. 



t The name syenite belongs by right of priority to the quartz-bearing 

 rock, as it was described from the locality Syene in Egypt, where that kind 

 occurs ; and I therefore call the kind free from quartz hyposyenite. Syenite 

 is a rock of the hornblendic series in all its geological relations, gradua- 

 ting often into hyposyenite in Archaean regions ; and it is bad to make 

 it, as some German lithologists do, a hornblendic variety of granite. It 

 deserves to stand as a distinct species • and it naturally leads off the horn- 

 blendic or syenitic series of crystalline rocks, as granite does the non- 

 hornblendic or granitic series. 



