TERTIARY AND VOLCANIC. 



9 



which the surface of the globe had been much fractured and displa- 

 ced ; for the upper series do not lie parallel with the lower, but they 

 cover the edges of the lower strata unconformably. 



To make this better understood, suppose a number of books to be 

 laid regularly upon each other, and the lowest volume to be tilted up 

 so as to give an inclined position to the whole, if we then take other 

 books and place them horizontally, or nearly so, on the upper edges 

 of the inclined volumes, we may then form a distinct idea of the un- 

 conformable position of the upper series of secondary strata over the 

 lower series. This position is represented Plate 1 . fig. 3. ; it will 

 be more fully described in the 4ih chapter.* The last of the upper 

 secondary strata is chalk, a rock well known in the south and south- 

 east parts of England, though entirely wanting in the north-west and 

 in Scotland. 



Tertiary Strata comprise all the regular beds that have been de- 

 posited subsequently to the chalk strata, on which they frequently re- 

 pose. It was formerly supposed that tertiary strata were very limit- 

 ed in extent, and were confined to a few districts in Europe ; recent 

 observations, however, prove that strata of this class cover consider- 

 able portions of the surface in various countries, though there are 

 other countries in which they are entirely wanting. Tertiary strata 

 are the most recent or uppermost of all the regular rock formations. 

 They consist chiefly of clay, marie, limestone, and friable sandstone : 

 the lower series of these strata contain numerous marine shells, while 

 some of the middle and upper strata contain shells resembling those 

 found in our present rivers, or in fresh water lakes. The most re- 

 markable fact respecting the tertiary strata is, that some of them 

 contain numerous bones of large terrestrial quadrupeds of the class 

 Mammalia, the greater part of which, belong to genera or species 

 which no longer exist upon the earth. 



Volcanic and Basaltic Rocks have been either ejected from vol- 

 canoes, or poured out in a slate of fusion from rents and openings on 

 the earth's surface. They cover in an irregular manner the rocks 

 of the preceding classes. In some situations the melted mineral mat- 

 ter has taken a columnar form in cooling ; in other situations it fills 

 vast fissures, called by miners dykes. Basaltic rocks are very com- 

 mon in the northern part of our island. Volcanic and basaltic rocks 

 are of different ages : the most ancient approach in their nature to 

 rocks of the primary class, and appear to be formed chiefly of the 

 same mineral snbstances, more or less softened by subterraneous heat, 

 and protruded through the crust of the globe. 



* There are some siUiatior>s in which the lower strata have not been subjected 

 to any great dislocations prior to the deposition of the. upper strata upon them, fop 

 the latter occur in a position parallel with that of the lower strata. 



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