T E R 



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T E R 



terroso, It.) Consisting of earth 

 earthy. 



TERRE'STRIAL. (terreslris, Lat. terrestre 

 Fr. terrestre, It.) Pertaining to the 

 earth ; consisting of earthy matter. 



TERRE'STRIAL REFRA'CTION. It is the 

 power which air possesses, in common 

 with all transparent media, of refracting 

 the rays of light, or bending them out ol 

 their straight course, which renders a 

 knowledge of the constitution of the at- 

 mosphere important to the astronomer. 

 Whenever a ray of light passes obliquely 

 from a higher level to a lower one, or 

 vice versa, its course is not rectilinear, 

 but concave downwards ; and of course 

 any object seen by means of such a ray, 

 must appear deviated from its true course, 

 whether that object be, like the celestial 

 bodies, entirely beyond the atmosphere, 

 or, like the summits of mountains, seen 

 from the plains, or other terrestrial sta- 

 tions, at different levels, seen from each 

 other, immersed in it. Every difference 

 of level, accompanied, as it must be, with 

 a difference of density in the aerial strata, 

 must also have, corresponding to it, a 

 certain amount of refraction. This re- 

 fraction between terrestrial stations is 

 termed terrestrial refraction. The re- 

 fraction of a terrestrial object is estimated 

 differently from that of a celestial body. 

 It is measured by the angle contained 

 between the tangent to the curvilineal 

 path of the ray where it meets the eye, 

 and the straight line joining the eye and 

 the object The quantity of terrestrial 

 refraction is obtained, by measuring con- 

 temporaneously the elevation of the top 

 of a mountain above a point in the plain 

 at its base, and the depression of that 

 point below the top of the mountain. 

 The distance between these two stations 

 is the chord of the horizontal angle ; and 

 it is easy to prove that double the refrac- 

 tion is equal to the horizontal angle, in- 

 creased by the difference between the ap- 

 parent elevation and the apparent de- 

 pression. Whence it appears that in the 

 mean state of the atmosphere, the refrac- 

 tion is about the fourteenth part of the 

 horizontal angle. Mrs. Somerville. Sir 

 John Herschel. 



TE'RTIARY. Third. A term applied to 

 those formations which have been depo- 

 sited subsequently to the chalk formation. 



TE'RTIARY STRA'TA. A division of sedi- 

 mentary formations called tertiary, as 

 being of newer origin than the secondary, 

 and characterized by distinct species of 

 fossil animals and plants. They present 

 a most decided contrast with the second- 

 ary and older strata in most of their es- 

 sential characters. The most striking 

 feature of these formations consists in the 



repeated alternations of marine deposites 

 with those of fresh water. We are in- 

 debted to Cuvier and Brongniart, for the 

 first detailed account of the nature and 

 relations of a very important portion of 

 the tertiary strata, namely, those which 

 occur in the neighbourhood of Paris. 

 These were found to fill a depression in 

 the chalk, and to be composed of different 

 materials, sometimes including the re- 

 mains of marine, sometimes of fresh- 

 water animals. The first discovery of the 

 tertiary strata in the Isle of Wight and 

 south-east of England, .we owe to Mr. 

 Webster. The whole of the tertiary ac- 

 cumulations are stratiform deposites, ex- 

 hibiting various kinds of lamination and 

 bedding. Previously to the commence- 

 ment of the present century, the true 

 nature of the tertiary formations was 

 unknown, the chalk was considered the 

 highest known rock, and the tertiary de- 

 posites as mere superficial sands, gravels, 

 or clays. 



The tertiary system may be said to con- 

 stitute a series of formations which link 

 together the present and the past ; while 

 the more ancient tertiary deposites con- 

 tain organic remains related to the se- 

 condary formations, the most recent con- 

 tain many existing species of animals and 

 plants, associated with forms now ex- 

 tinct. The English series of marine 

 tertiary deposites is most distinctly shewn 

 in the Hampshire basin, and the Isle of 

 Wight, in the London basin, and on the 

 eastern coast, from the mouth of the river 

 Yare to that of the Thames. In France, 

 however, we find a much more full de- 

 velopment than in England of the ter- 

 tiary strata. From the commencement 

 of the tertiary period, there appears to 

 have been a constantly increasing pro- 

 vision for the diffusion of animal life. 

 The term tertiary has been altogether dis- 

 approved of by some geological authors. 

 Mr. De La Beche considers the name 

 highly objectionable, " inasmuch as it 

 would imply that there were three great 

 classes of rocks possessing marked cha- 

 racteristic distinctions, and that the de- 

 posites above the chalk constituted the 

 third of such classes." He proposes 

 that the term supra-cretaceous be em- 

 ployed in preference to tertiary. On the 

 other hand, Mr. Bakewell says, "the 

 name of tertiary has been given with much 

 propriety to all the strata that are more 

 recent than the secondary ; the term is 

 intelligible, and ought not to be changed 

 without sufficient reason. The name of 

 supercretaceous, which has recently been 

 applied to the tertiary strata, is peculiarly 

 inappropriate, as these strata may cover 

 any of the lower rocks, and in Auvergne 



