EAR 



[147] 



E C H 



whatever the miner may reveal in 

 artificial excavation ; but the whole 

 of that outer covering of the planet 

 on which we are enabled to reason 

 by observations made at or near 

 the surface." 



It has been concluded, both from 

 astronomical and geodesical obser- 

 vations, that the figure of the 

 earth is a spheroid. This spheroid 

 has been considered as one of 

 rotation, or such figure as a fluid 

 body would assume if possessed of 

 rotatory motion in space. The 

 amount of the flattening of the 

 poles, or the difference of the 

 diameter of the earth from pole to 

 pole, and its diameter at the 

 equator, has been variously esti- 

 mated, the commonly received 

 opinion is, that the polar axis is to 

 the equatorial diameter as 304 to 

 305, the difference in favour of the 

 equatorial diameter amounting to 

 26 miles. As regards the density 

 of the earth, various opinions have 

 been formed; it however appears 

 certain, that the internal exceeds 

 that of the solid superficial density. 

 Daubuisson infers from the obser- 

 vations of Maskelyne, Cavendish, 

 and Playfair, that the mean density 

 of the earth is above five times 

 greater than that of water, and 

 consequently, about twice that of 

 the earth's mineral crust. Laplace 

 calculated the mean density to be 

 1'55, the solid surface being 1. 

 Eaily, in his astronomical tables, 

 states the mean density of the earth 

 to be 3*9326 times greater than 

 that of the sun, and to that of 

 water as 11 to 2. 



A very great proportion of the 

 earth's surface bears the most in- 

 contestible evidence of having been 

 deposited under water ; of having 

 been, at one time, not only sub- 

 merged by the sea, but gradually 

 accumulated at its bottom. Nearly 

 ail the strata contain the remains 

 of shells in great profusion, the 



bones and scales of fishes, fragments 

 or entire trunks of timber, frag- 

 ments also of stone, very commonly 

 in the form of pebbles, evidently 

 worn and rounded by friction 

 under water. 



The term " earth's crust" relates 

 only to the comparative extent of 

 our knowledge beneath its surface, 

 and is not used with the intention 

 of conveying an opinion that the 

 earth consists only of a crust, or 

 that its centre is hollow; for of 

 this we know nothing. The 

 nature of the earth's crust is most 

 readily studied in mountains, 

 because their masses are obvious; 

 and also because, as they are the 

 chief depositories of metalliferous 

 ores, the operations of the miner 

 tend greatly to facilitate their 

 study. 



EBOU'LEMENT. (Fr.) Fall of any 

 detached rock. The fall of parts 

 of mountains is so common an oc- 

 currence in the Alps, that it is 

 expressively called an eboulement, 

 from the verb ebouler ; tomber en 

 ruine. 



EBU'BNA. (from eburnus, Lat. ivory.) 

 An oval or elongated univalve with 

 a deeply umbilicated columella; 

 the aperture oblong, and notched 

 at the bottom. The recent eburna 

 lives in sandy mud. Fossil eburnae 

 are rarely met with. Parkinson 

 states that Lamarck does not notice 

 them among the Paris fossils, but 

 that a shell exists among the Essex 

 fossils which he names Eburna 

 glabrata. Dr. Mantell gives eburna 

 as a fossil of the chalk marl, but 

 affixes a note of interrogation to it, 

 as though doubtful. 



ECHI'NIDEA. An order of the class 

 Echinodermata, and divided into 

 three families, viz., Spatangoidea, 

 Clypeasteroidea, and Cidarida3. 



ECHIDNIS. A fossil resembling an 

 orthoceratite, its specific character 

 being the alternate circular risings 

 and depressions on its surface. . 



