957 



GENUS. 



GEOLOGY. 



958 



form part of that of the Straits of Magellan. The most common genus 

 is Gtntiana, than which few genera display so full a series of colours 

 in the flowers ; red, blue, yellow, and white are all exhibited in it, 

 with many of the intermediate compound tints. For the properties 

 of the genera of this order see CICENDIA, CHLORA, MENYANTHES, 

 VlLLABSIA, AllATHOTES. 



GENUS, an assemblage of species allied by common characters, and 

 subordinate to an order, family, tribe, or sub-tribe. A genus is fre- 

 quently a natural assemblage, but almost every naturalist has his own 

 particular views with regard to the propriety of uniting or separating 

 particular groups of species ; it is therefore often an arbitrary group. 

 It is thus that the synonymy of genera becomes every day more 

 copious, and it is one of the difficulties with which the naturalist has 

 to contend in his studies. At present there are no generally recognised 

 rules for the construction of genera amongst plants and animals. 

 [FAMILIES OF PLANTS ; ORDERS ; SPECIES.] 

 GEOBDELLA. [ANNELIDA.] 

 GEOCICHLA, a genus of Birds established by Mr. Gould for a 

 pretty species resembling the Redbreast (Erithacus Rubecula, Swain- 

 son). It belongs, he observes, to an interesting group which was 

 first characterised by M. Kuhl, and of which the collection of the 

 Zoological Society possesses four well-marked species. (' Zoological 

 Proceedings,' 1836.) 



GEOCOCHLIDES, Latreille's name for the Shell-Snails. Trache"- 

 lipodes Colimaces of Lamarck ; Limacine's of De Blaiuville ; Limacons 

 of De Ferussac. 



GEOCRONITE, a Mineral occurring amorphous without cleavage. 

 The fracture is lamellar in one direction, and in the other granular 

 and conchoidal. The colour lead-gray ; streak the same. Hardness 

 between mica and calcareous spar. Lustre metallic. Opaque. Specific 

 gravity 5'88. It ig found in the silver-mine of Scala in Sweden, and 

 in the province of Galicia in Spain. An analysis of the mineral from 

 the Scala mine by Svanberg gives 



Lead 66'452 



Antimony . 9-516 



Arsenic 4-695 



Copper . 1-514 



Iron 0-417 



Zinc . 0-111 



Sulphur 16-262 



GEMOEYD^. [CHELOMA.] 

 GEOFFR^EA, a genus of Plants belonging to the natural order 

 Leyuminoste. One of ttie species, G. inermii, sometimes known as 

 Andira incrmit, a a native of Jamaica, Trinidad, Martinico, Porto 

 Uico, St. Domingo, and Guyana, in woods and on river-banks. It has 

 13 or 15 ovate-lanceolate leaflets, acute, glabrous on both surfaces, 

 the flowers pannicled on short pedicles ; calyx urceolate, clothed with 

 rusty pubescence. The flowers are arranged in terminal and axillary 

 ferruginous panicles, very showy, with reddish lilac petals. The 

 legume is the size of a large plum. 



The bark of this tree is of a grey colour externally, but black and 

 furrowed on the inside. The powder looks like jalap. It has a 

 mucilaginous and sweetish taste, and a disagreeable smell. Its 

 medical effects are great. When properly exhibited it operates as a 

 powerful anthelmintic. It is given in the form of a powder, decoc- 

 tion, syrup, and extract, but should always be given in small doses ; 

 in large doses it is poisonous, producing violent vomiting, with fever 

 and delirium. 



GEOLOGY, the science of the earth (as the Greek words 77) and 

 \lrfOt may be translated), includes, in a large sense, all acquired or 

 possible knowledge of the natural phenomena on and within the globe ; 

 whether these be now of frequent occurrence, the result of the exist- 

 ing combinations of physical agencies, or remain as monuments and 

 measures of those agencies in earlier periods of the history of the 

 planet. 



Some of these phenomena are witnessed in connection with inorganic 

 bodies, and depend in a great degree on the laws of force which apper- 

 tain to and distinguish from each other the particles of matter ; others 

 are exemplified in organised structures endowed with vital functions 

 related to those structures ; and there may yet be distinguished a 

 third order of effects, influencing and combining with both of the 

 former, and depending on laws of force which affect the whole mass 

 of the globe, as gravitation, or derived from extraneous agency, as 

 ligh't. 



If at any certain epoch (as the present time) the phenomena thus 

 classed were known in detail, and reduced to general laws which 

 truly expressed the individual cases, the actual condition of the earth 

 would be really known ; if further it were possible to collect sufficient 

 evidence from monuments preserved in the earth of its exact state at 

 some former epoch, the variations to which terrestrial phenomena are 

 subject would be disclosed ; and by the comparison of several such 

 surveys, taken at distant times, the laws of these variations would be 

 revealed with an exactness proportioned to the certainty with which 

 the intervals of time were determined. These laws of the variation 

 of the condition of the globe at successive epochs, combined with the 

 laws of chemical, vital, and mechanical action, which are assumed to 

 be essential and constant, independent of time, and exempt from 

 change, will furnish one, and only one, satisfactory general contem- 



plation or theory of the origin, structure, and successive changes of 

 the globe, considered as part of the planetary system revolving round 

 the sun. 



To reach this general theory is the ultimate object of modern 

 geology. The discovery of the right method of proceeding in this 

 attempt is of modern date ; and all the most important steps of the 

 advance towards this ' high point of knowledge ' have been taken 

 within the memory of the generation now passing away. If, as Sir 

 John Herschel tells us (' Discourse on the Study of Natural Philo- 

 sophy '), " geology, in the magnitude and sublimity of the objects of 

 which it treats, undoubtedly ranks, in the scale of the sciences, next 

 to astronomy," it owes this distinction to the fact that its modern 

 cultivators have sought within the ranks of inductive science better 

 methods of research and purer models of reasoning than those afforded 

 by the treasures of ancient philosophy which have been preserved to 

 our time. Nor is this the peculiar boast of geology. Every branch 

 of the study of nature was equally transformed by the introduction 

 of the Baconian methods of interpretation of nature ; all the natural 

 sciences have advanced together ; the knowledge of the constant laws 

 in the visible creation has been continually perfected ; and thus, 

 while the study of the long-past operations of nature has been imbued 

 with the exactness of chemical, zoological, botanical, and physical 

 research, the dry annals of one era in the history of the world have 

 been enriched into a long, instructive, and eventful history. 



Among the ancients the notices of geology are few, and the interest 

 belonging to them is of a peculiar character. When chemistry, whose 

 operations manifest the existence of peculiar laws of force among the 

 particles of matter, was wholly unknown when the living wonders 

 of creation were but slightly considered by philosophers intent on 

 abstract principles no accurate survey could be taken of the con- 

 dition of any one part of the surface of the earth. But a small part 

 of that surface was known to any one people, and only in a few 

 situations were the changes in the aspect of nature so extensive as to 

 arrest the attention of the geographer, or so violent as to excite the 

 philosopher to search for the cause. 



Among the anciently peopled and commercial states of the eastern 

 shores and islands of the Mediterranean both these circumstances 

 concurred, and there first awakened the powerful intellect of Greece 

 to speculation on the varying condition of the land and sea. Lower 

 Egypt is the gift of the Nile, and the learned people which possessed 

 it were compelled by the circumstances of'their situation to study the 

 nature and effects of the annual floods of the river. Herodotus (born 

 B.C. 484) estimates (ii. 11) that the Nile, if diverted into the Red Sea, 

 would fill that long gulf in less than 20,000 or even 10,000 years. 

 The notion of change thus distinctly impressed upon the minds of 

 the Egyptian priests was developed in a general and philosophical 

 form, and illustrated by special references to an extended series of 

 geological phenomena by their pupil Pythagoras (born B.C. 586). 

 According to the summary of their doctrine, and the tenor of the 

 illustrations of it which are given by Ovid, we cannot avoid seeing, 

 even through the injurious ornament of verse, that Pythagoras had 

 acquired a clear conception, a ' distinct idea,' of nature as existing by 

 the concurrent action of many complicated powers, which were subject 

 to continual or sudden variation in their relative intensity. Changes 

 of the relative level of land and sea, and divulsion of islands from the 

 mainland by the action of earthquakes, are distinctly announced ; the 

 displacement and limited duration of volcanic vents, such as /Etna; 

 the degradation of land by the action of atmospheric agency (' et eluvie 

 mons est deductus in asquor;') the submersion of land which had 

 been formerly peopled 



Si quajras Helicen et Burin, Achaidas urbes, 

 Invenies sub aquis. (Ovid, ' Metam.' xv., 1. 293.) 



the productioD of new land, and the occurrence of marine shells far 

 from the present seas ; these phsenomeua, distinctly observed and 

 analysed, and clearly produced in proof of a general proposition, 

 justify a higher degree of admiration for the Samian philosopher 

 than is due to any of the merely speculative writers of antiquity. 



Similar observations appear to have served as the ground-work of 

 Aristotle's exposition (' Meteorologica ' ) of the perpetual fluctuation 

 of natural phenomena ; the alternate excitation and rest of parts of 

 the earth's surface. But it is in Strabo that we find the most 

 sensible views of the causes of the occurrence of marine shells far 

 from the shore, the displacements of land and sea, the rising of 

 islands, the formation of straits, and other great geological phe- 

 nomena. 



Having stated the views of Eratosthenes, as to the general fact of 

 the earth's globular form, and the production of the numerous minor 

 inequalities on its surface, by correspondingly numerous ' proximate 

 causes," such as the operations of water, heat, concussions, vapours, 

 and the like, he examines the opinions of Xanthus and Stratou, 

 which Eratosthenos had preserved. (Strabo, Casaub. 49, &c.) 



The explanation of Xanthus (derived from an historical fact) that 

 the phenomena in question were due to great droughts which had 

 diminished the originally greater expanse of the sea, is regarded as 

 insufficient ; and Strabo's hypothesis of adjacent but disconnected 

 seas, ont of which being raised to a higher level by sediment on its 

 bed, had forcibly opened itself a passage to the other, the Euxine to 

 the Propontis, the Mediterranean to the Atlantic, is shown to contradict 



