613 



ROCHEA. 



ROCKS. 



614 



S. linctoria., Dyer's Roccella, or Orchil. Thallus suffruticose, 

 rounded, branched, somewhat erect, grayish-browii, bearing powdery 

 warta; apothecia flat and horny, with a scarcely prominent border. 

 A practical writer describes " the good kind as having a nearly white 

 powder on its surface towards the centre ; the under surface is of a 

 gray colour, and is not hairy ; if wetted, it does not turn of an orange 

 colour ; its edges are flat and thin." 



R. fuciformig, the Flat- Leaved Orchil. Thallus flat, branched, nearly 

 upright, grayish-white, bearing powdery warts; apothecia horny, 

 bordered. 



Both kinds are found on maritime rocks, or on dry stone walls 

 exposed to the influence of the sea-breeze, aa well on the coast of 

 England as on the shores of the Mediterranean and the East Indies. 

 [ORCHIL, in ARTS AXD Sc. Drv.] The more arid the situation, the 

 better is the quality of the lichens. The presence of the colouring- 

 matter is ascertained by steeping the weed broken up in small pieces 

 in diluted solution of ammonia, in a bottle half filled with liquid, 

 which should be kept corked, but frequently opened in a temperature 

 not exceeding 150 Fahr. 



1, Koecella tinetoria; a, warts on the thallus. 2, Kocctlla linctoria (East 

 India). 3, Hocctlla fuci/ormii ; i, apothecia. 



ROCHEA (named after La Roche, a French botanist), a genus of 

 Plants belonging to the natural order Crasiulaccce. It has a 5-lobed 

 calyx ; petals 5, united into a gamopetalous hypocrateriform corolla, 

 with a short tube, equal in length to the spreading limb or shorter 

 than it; stamens 5, alternating with the petals, a little exserted; 

 glands and carpela 5. The species are fleshy simple succulent shrubs. 

 The leaves opposite, connate at the base, thick, and white. The 

 flowers are disposed in terminal corymbs, without any bracts. A 

 large number of the species are cultivated in our gardens and green- 

 houses. 



ROCINELLA. [IsoPODA.] 



HOCK-CRYSTAL. [QUARTZ.] 



ROCK-FISH, a name for the Black Goby. [GoBius.] 



ROCK-MANAKIN. [Rupicous^.] 



ROCK-PLANTS are those plants which are distinguished by 

 growing on or among naked rocks, and are confined to no particular 

 region or latitude. De Candolle (' Dictionnaire des Sciences Natu- 

 relle*,' vol. xviii.) observes that they pass by insensible gradations into 

 the plants that inhabit walls, rocky and stony places, and even gravelly 

 places, from which they pass into those that are found particularly on 

 lands and barren soils. Most of the plants growing on rocks have 

 but a small development of root, as they derive their nutriment prin- 

 cipally from the air, through the medium of their leaves and stems. 

 A large number of the Cryployamia, especially Mosses and Lichens, 



belong to this class. Many of these plants present a remarkable 

 diversity, according to the nature of the rocks on which they grow. 

 [VEGETABLE KINGDOM.] 



ROCK-ROSE. [Cisius.] 



ROCK-SALT. [SODIUM.] 



ROCK-SNAKE. [Boms.] 



ROCKET. [ERUCA.] 



ROCKET, SEA. [CAffiLE.] 



ROCKET, YELLOW. [BARBAREA.] 



ROCKLING. [MOTELLA.] 



ROCKS, in Geology, are any aggregation or collection of minerals 

 or fragments of mineral matter, whether they are crystalline or 

 amorphous, hard or soft, compact or loose. On this account they 

 vary exceedingly in character, and no simple definition will give an 

 idea of a rock as distinct from other objects around. The causes 

 which produce them are the great agents of all geological change. 

 They may be merely mechanical heaps, chemical crystallisations, and 

 contain in them organic beings or not. Their exceedingly mixed 

 character makes it difficult to classify them, independent of their 

 age, as arranged in the various geological strata. [GEOLOGY.] 



The most prevalent classification of rocks ill actual use is founded 

 on one leading feature of their origin and history. Rocks are of 

 igneous origin (Pyrogenous Rocks), or of aqueous origin (Hydrogenous 

 Rocks), and thus make two great classes ; the former being often con- 

 sidered, with reference to the circumstances of their occurrence in 

 two divisions, namely, hypogene or Plutonic rocks (as granite), and 

 volcanic rocks (as obsidian) ; the latter being distinguished into fresh- 

 water and marine deposits, the result of chemical, vital, and mechani- 

 cal agencies exerted in water. Many cases are known of an alteration 

 of these hydrogenous rocks by contact with Plutonic or volcanic 

 masses : by this change they acquire the name of Metamorphic Rocks. 

 Thus clays and sandstones are hardened, and have their structure 

 altered so as to resemble clay-slate, quartz-rock, or jasper; and chalk 

 and limestone are rendered crystalline. 



Adopting as the best and most applicable the fundamental distinction 

 of pyrogenous and hydrogenous rocks, the student will find by expe- 

 rience that the best if not the only good way of describing and recog- 

 nising rocks, is by attending to their elementary composition. The 

 granitic, porphyritic, amygdaloidal, or other structures of igneous 

 rocks, are so many variable circumstances due to particular accidents 

 in the fusion or cooling of the masses, and belong more or less to all 

 of them, as the compact, oolitic, arenaceous, and other characteristic 

 textures of hydrogenous rocks, mark peculiarities of their aggregation 

 or solidification. 



Mr. Scrope has successfully shown, in his ' Synopsis of Volcanic 

 Rocks' ('Journal of Science, vol. xxi.), that these various compounds 

 may bo easily and philosophically classed by consideration of the 

 relative abundance of two minerals seldom absent from any of them, 

 namely, felspar and augite. (The felspar is sometimes replaced by 

 leucite, hailyne, olivine, or mellite; the augite by hornblende or 

 titaniferous iron.) Hence we have only three great groups : 



Felspathio .... Trachyte. 

 Augito-Felspathic . . . Graystone. 

 Augitic Basalt. 



To each of these belong many varieties, and many gradation^ of 

 granitic, porphyritic, amygdaloidal, and other structures. [LAVA.] 

 By a similar method we may class the older or Plutonic rocks of 

 fusion, as : Felspathie granite, porphyry, felspar rock, claystone, 

 eurite, pitchstone; Augito-Felspathic sienite, euphotide; Augitic 

 sienite, hypersthene rock, greenstone, basalt, wack<5, melaphyre. 



The hydrogenous rocks of most importance may be classed according 

 to the'r arenaceous, argillaceous, calcareous, or other basis, as : 



Arenaceous, uniform, as sandstones, sands; aggregated as conglo- 

 merates, pudding-stones. 



Argillaceous, uniform, as clay and shale ; containing fragments, as 

 some clay conglomerates. 



Argillo-Calcareous, as marls properly so called. 



Calcareous, as chalk, limestone. 



Calcareo-Magnesiaa, as dolomite. 



Haloid, as gypsum, rock-salt. 



Carbonaceous, as coal, lignite. 



Ferruginous, as ironstone. 



Cupriferous, as the kupferschiefer. 



Finally, all these hydrogenous rocks are liable ttf local changes, by 

 contact or proximity with the rocks of fusion. These metamorphic 

 rocks may bo classed according to the same form as those which are 

 unaltered. We give below authentic examples of several cases of 

 metamorphism : 



1. Arenaceous Rocks, metamorphic by induration, as along green- 

 stone dykes in Arrau, and Salisbury Crags. Similar effects happen 

 beneath iron-furnaces, and when the effect is in extreme the result is 

 quartz rock. 



2. Argillaceous Rocks, metamorphic by induration, and a certain 

 confluence of grains. In extreme cases the result is a kind of clay- 

 slate, or hone-slate, or Lydian stone, with cubic pyrites, and rarely 

 garnets, imbedded. An example of the latter occurs at Plas Newydd. 



3. Calcareous Rocks, metamorphic by re-arrangement of particles; 



