MINERALOGY. 



399 



Reognr. wail of coral for the most part, in sitmtions where the 

 - -,- J winds are con-.tnnt. being arrived at the surface, affords 

 a shelter, to leeward of which their infant colonies may 

 be safely sent forth ; and to this their instinctive fore- 

 t-ms to be owing, that the windward side of 

 a reef e\prKi-l to the open sea, is generally, if not al- 

 wsv. the highest part, and rises almost perpendicular, 

 sometimes from the depth of 200, and perhaps many 

 more fathoms. To be constantly covered with water, 

 seetns necessary to the existence of the animalcules, 

 far they do not work, except in holes upon the reef, 

 beyond low-water mirk ; but the coral sand and other 

 broken remnants thrown up by the sea, adhere to the 

 rock, and form a solid mass with it, as high as the com- 

 mon tides reach. That elevation surpassed, the future 

 remnants, being rarely covered, lose their adhesive pro- 

 perty ; and remaining in a loose state, form what is 

 usually called a key, upon the top of the reef. The 

 new bank is not ion;; in being visited by sea birds ; 

 salt plants take root upon it, and soil begins to be 

 formed; a cocoa-nut, or the drupe of a pandanu*, is 

 thrown on shore ; land birds vi.it it, and deposit the 

 seeds of shrubs and trees ; every high tide, and still 

 more every pale, add* something to the bank ; the form 

 of an island is gradually assumed, and last of all comes 

 man to take possession. 



Half-way I .land is well advanced in the above 

 progressive state : having been many years, probably 

 some ages, above the reach of the highest "pring tides, 

 or the wash of the surf in the heaviest gales. 1 distin- 

 nttbed, however, in the rork which forms its basis, 

 the sand, coral, and shells, formerly thrown up, in a 

 more or less perfect state of cohesion. Small piece* of 

 wood, pumice stone, and other extraneous bodies which 

 chance had mixed with the calcareous substances when 

 the cohesion began, were inclosed in the rock ; and in 

 some cases were still separable from it without much 

 force. The upper part of the island is a mixture of the 

 same substances in a loose state, with a little vegetable 

 oil ; and is covered with the catuarina and 

 of other tree* and shrubs, which give food to parro- 

 queta, pigeons, and some other In-'-, to whose ances- 

 tors, it is probable, the island was originally indebted 

 for this vegetation." 



CHAP. II. 



EFFECTS OF WATER AND VOLCANOES ON THE SURFACE 

 OF THE EARTH. 



Effect* at 29. Having now described all the varieties of ine- 

 wMtrr nd quality observed on the surface of the dry Und, it will 

 volcnoci be interesting to endeavour to discover how these have 



tarfTbV bwn forme<l - 



.;,.'. Many of the elevations and hollows appear to be ori- 



ginal formations, while others seem to be more parti- 

 cularly the effect* of water and of volcanic tire. \Vc 

 shall consider each of these in particular. 



30. Water acts either chemically or mechanically on 

 the surface of the earth : and it may further be consi- 

 dered, according to its destroying and forming effects. 



Deilroying Effeelt of Water. 



Mtckanical We shall first consider the mechanical dc*rmn*e. tffrclt 

 **"')' of 'water. 



Every long continued rain convinces us of the pow- 

 erful mechanical effect* of water on the surface of the 

 earth. The precipitated water penetrate* the surface, 



7 



then flows along mixed with the matter of the soil, and Geognosy- 

 in its course forms small water-courses, and occasions """" "Y"~* 

 considerable changes in the flat country and the decli- 

 vities of mountains. As it rains a very considerable E ff ects o 

 portion of the year, and as every rain carries along with " e * nd 

 it a quantity of the soil, very obvious changes must in tpoutfc 

 this manner be induced on the surface of the globe. 

 Thunder storms and water-spouts, although more un- 

 common phenomena, produce more considerable chan- 

 ges, and this either alone, or when their waters join or 

 flow into rivers. These deprive whole districts of their 

 soil to the bare rock ; they sometimes even form small 

 ravines, and break down and carry away great masses 

 of rock, that were either formerly much rent, or of such 

 a form as to be easily overpowered by water. If such 

 changes take place in the low land, they must be vast- 

 ly more considerable in the high land. 



The thaw-floods that take place in low countries to- Effect* of 

 wards the end of winter and beginning of spring, and thaw- 

 in mountainous districts during summer, occasion still fl oo<l- 

 greater changes on the surface of the earth. Their ef- 

 fects are truly frightful, particularly when accompanied 

 with rain. The declivities in low countries, over 

 which water flows, are less considerable than in high 

 countries ; and lifides, the water can extend Itself far- 

 ther in low and flat countries ; hence its destroying 

 effects are diminished in intensity. In mountainous 

 countries, on the contrary, the fall is much greater 

 than in flat countries, and the water is compressed 

 into narrow rocky valleys ; hence it follows, that rain- 

 floods must be more destructive, the more considerable 

 the quantity of water, and the more mountainous the 

 country. These floods are still more destructive, when 

 the mountain rocks are of such a nature, as to afford 

 little resistance to the impetuosity of the water ; that 

 is, when they are decomposed, loose in their texture, 

 or have such a shape as to allow the water to act more 

 easily on them. If we compare together all these cir- 

 cumstances, we shall find that mountainous countries 

 are more liable to suffer from the effects of floods, than 

 low and flat countries. To this, indeed, there are ex- 

 ceptions, as in the case in some granites, and other rocks 

 that long resist the effects of the most powerful and 

 violent flood*. 



The water of these floods, in its progress towards 

 the lower parts of the earth, flows either into ravines, 

 and from these into valleys and beds of rivers ; or when 

 it meets with no ravine, scoops out a bed for itself, 

 wherever it meets with a soft yielding rock or a slight 

 hollow. The junction of these mountain-streams with 

 the river of the district not only increase* it* power 

 by the addition of a considerable quantity of water, 

 but also causes it to overflow its banks, and deluge the 

 neighbouring country, and thus to occasion great 

 changes on its surface'. The different loose materials 

 are carried towards the sea, and are deposited at 

 different distances from the mouth of the river; and 

 tbm are proportioned to the magnitude of the masses. 

 The finest or loamy parts reach the sea; the sand, 

 gravel, and larger rolled masses being left on the sur- 

 face at greater or less distance from die sea, according 

 to the relative magnitude of their part*. 



This mechanical action of water appears in many Formttion 

 caseg to have contributed in an eminent degree to the of valleys, 

 hollowing out of valleys ; but all valleys have not been 

 formed in this manner ; for many and very extensive 

 valleys are formed by mountain groups disposed in a 

 circular form.aj i* the case in Bohemia, Hungary, Tran- 

 sylvania, &c. ; others by the original inequalities of the 



