420 



MINERALOGY. 



Geognosy. 



Name. 



VI. Primitive Trap. 



Ur-Trap. Werner. 



Primitive Trap. Jameson. 



Granitelles, Trap, Corneennes. Sattsssurc. 



Trapps Primitifs. Brochant. 



Amphibolite. Daubuisson. 



1. Name. The name trap is derived from the Swe- 

 dish word trappa, signifying a stair. It would appear 

 that this name was first used by Rinman, in a memoir 

 on ferruginous stones, published in 1754. The Swedes 

 applied this name to rocks, which, on exposure to the 

 air, assumed shapes resembling the steps of a stair. It 

 was, however, soon extended to a considerable variety 

 of rocks of very different formations ; hence Werner 

 found it necessary to restrict its signification. He un- 

 derstands by trap, rocks principally characterized by 

 the presence of hornblende and black iron-clay. Hence 

 all rocks occurring in the primitive class, having horn- 

 blende as a characteristic or predominating ingredient, 

 belong to the Primitive Trap Formation. 



Varieties. 2. Varieties. Hornblende occurs in trap rocks, either 

 alone or mixed with other minerals, and having different 

 structures ; and this arrangement affords a good basis 

 for their subdivision. In the oldest trap, no iron- clay 

 occurs ; it first makes its appearance in the transition 

 period, and increases in the newer periods. 



Primitive Trap, in particular, is almost always dis- 

 tinguished by a great predominance of hornblende, 

 so that some of the kinds are wholly or almost entirely 

 composed of hornblende. This character affords the 

 first subdivision of primitive trap. 



There are three principal species of primitive trap, 

 and these again have their subordinate kinds. 



The following table exhibits the rocks of this series. 



1 . Common hornblende rock. 



. Granular hornblende rock. 

 b. Horn blende- Slate. 



2. Hornblende mixed with felspar. 



a. Greenstone. Diabase. Brongniarl. 

 <*. Common Greenstone. 



. ft. Porphyritic Greenstone. 

 y. Greenstone Porphyry. 

 3. Green Porphyry. 



b. Greenstone Slate. Diabase schistcnse. 



3. Hornblende mixed with mica. 



The three principal species are, 1. Common Horn- 

 blende-rock ; 2. Hornblende mixed ivilh felspar ; and, 

 3. Hornblende mixed with mica. 



Common 1. Cfimmon Hornblende-rock is almost entirely com- 

 horo- posed of hornblende. It contains two subordinate kinds; 

 blende- tne g rgt j s denominated Granular Hornblende-rock ; 

 the second, which differs from the first only in having 

 a slaty structure, is denominated Hornblende-slate. It 

 passes sometimes into gneiss, and sometimes into chlo- 

 rite-slate, and often into hornblende-rock. These two 

 rocks occur in beds, in gneiss, mica-slate, and clay- 

 slate, but the beds are thicker and more numerous in 

 the clay-slate than in mica-slate or gneiss. 



It occurs in the islands of Arran, Coll, and Tiree ; 

 also in the district extending from Loch Lomond to 

 Dunkeld, and many other places of the Highlands of 

 Scotland. It abounds also in Bohemia, Saxony, the 

 Tyrol, Siberia, and many other countries. 



Horn- 2. Hornblende mixed ivilk Felspar. This species con- 



blende tains two subordinate kinds; the first is, Greenstone, 

 mixed wiib tne se cond Greenstone-slate. 



felspar. ^ j yy j( . (greenstone comprehends the following va- 



rieties : Common Greenstone, Porphyritic Greenstone, 

 Greenstone- Porphyry, and Green Porphyry. 





a. Common Greenitone is a granular aggregate of Geognosy . 

 hornblende and felspar, b. Porphyrilic Greenstone is -"Y"""' 

 the preceding kind including lar^e crystals o!' fe!- 

 spar, and consequently having a porphyritic structure, 

 c. Greenstone Porphyry. In this variety the granular 

 basis, which is with difficulty distinguishable, includes 

 crystals of felspar. It is the Black Porphyry of the 

 ancients, d. Green Porphyry. In this variety the 

 granular nature of the basis is no longer visible to the 

 naked eye ; it appears uniform and simple ; has a 

 blackish green or pistachio green colour, and includes 

 crystals of compact felspar. It is the Par fire verle, or 

 antique green porphyry of antiquaries. The Variolite 

 probably belongs to the green porphyry. 



Greenstone appears sometimes stratified. Its dif- 

 ferent varieties first appear in gneiss, then in mica slate, 

 and lastly in clay-slate.. In mica-slate, but more par- 

 ticularly in gneiss, the beds are few and inconsiderable ; 

 whereas, in clay-slate, they are numerous and of great 

 magnitude. It probably, in some instances, occurs in 

 an unconformable and overlying position, and hence 

 may be sometimes considerably newer than clay-slate. 



It occurs abundantly in this country. Thus the clay- 

 slate and mica-slate that form so great a portion of the 

 country extending from Loch Lomond, by Callender, 

 Comrie and Dunkeld, contain numerous beds of green- 

 stone ; and there, as is the case in all other countries, 

 the clay-slate contains more numerous and larger beds 

 than the mica-slate. It is also very abundant on the 

 continent of Europe, as Norway, Saxony, Bohemia, 

 Silesia, Thuringia, Hungary, the Alps of Switzerland, 

 and Savoy. 



(2.) Greenstone-Slate is composed of hornblende and Green, 

 compact felspar, and has a distinct slaty structure. The stone-si 

 felspar in general is rather more abundant than the 

 hornblende. It sometimes contains scales of mica. 



It occurs only in clay-slate, and according to Werner 

 is the newest of the primitive traps. It occurs in great 

 beds, and even mountain-masses ; so that in some coun- 

 tries, as Sweden, it is said to form ranges of hills. It is 

 very metalliferous. The celebrated mining district of 

 Gersdorf, in Saxony, is situated in this rock. The 

 mining district of Rudolstadt in Silesia, and of Adelfors 

 in Sweden, are also in greenstone-slate. 



3. Hornblende mixed n-ith Mica, This is an intimate Horn- 

 mixture of hornblende and felspar, that includes scales blend* 

 of mica. It occurs, in beds, in gneiss and mica-slate, mixed wit 



mica. 



VII. Serpentine. 



Serpentiii. Werner. 

 Serpentine. Kirman. 

 Serpentine. Jameson. 

 Serpentine. Brocliant. 



1 . Minerals imbedded in it. It is to the eye a green co- Minerals 

 loured simple mountain rock, of which a description will embedded 

 be given in our account of Simple Minerals. It frequently "' " 

 contains accidental minerals, or is indeterminately mix- 

 ed with another mineral. Of the latter only one in- 

 stance is known. It is the mixture of limestone and 

 serpentine, forming what is denominated cerde an- 



tico. The accidental mixed minerals are common talc, 

 indurated lithomarge, steatite, common asbestus, ami- 

 anthus, mica, schiller-stone, native magnesia, magne- 

 site, meerschaum, actynolite, rock-cork, rock-wood, 

 diallage, pyrope, opal, chrysoprAse, hornsione, amethyst, 

 quartz, and hornblende crystals. 



2. Subordinate beds. The only beds it contains are gubordi- 

 limestone and euphotide. nate beds. 





