174 Analysis of Scientific Books. 



in much greater abundance than the stratiform formations on 

 which it reposes. But the very reverse of this is the case ; the 

 majority of observers agreeing in the fact, that basalts, and 

 the rocks of the same nature, contain no trace of organized 

 matter. Kirwan, indeed, and some German mineralogists, 

 have asserted the presence of shells in basalt and in wacke ; 

 but these shells, in number inconsiderable, and of undeter- 

 mined genera, have been observed only in a very few loca- 

 lities. And even should it be proved, which it has not, 

 that the rocks which contain them are true basalts and true 

 wackes, it would be no less certain that fossil shells are the 

 greatest rarities in this class of rocks. Mr. Necker affirms 

 that in those which he has himself observed in Scotland, 

 in Auvergne, in the Vivarais, and in Languedoc, he has 

 never perceived the slightest trace of organized bodies. The 

 preceding facts lead him to consider the basaltic rocks as a 

 product of dift'erent origin from the others, or a formation sui 

 generis, and he calls it accordingly the independent trap forma- 

 tion. In retaining for this class of rocks the generic title of 

 traps, which several German and French mineralogists have 

 given it, he desires it to be understood that the traps of the 

 Swedes, and of M. Faujas, do not belong to this division. In 

 fact, these latter are transition rocks, corncennes, varieties of 

 petrosilex, or siliceous schists, often even greywackes, of a very 

 fine grain and compact texture. He also excludes the rocks of 

 hornblende of the primitive formation, as well as the transition 

 grunstein, although Werner has grouped all these rocks under 

 the denomination of trap. In reality, a comparative examination 

 of the grunstein of transition, with the trap grunstein, discloses 

 marked differences in their composition and in their structure. 

 In the last, the hornblende exists in greater abundance ; it is 

 presented under the form of short and narrow plates, while, in 

 the first and in the primitive amphibolic rocks, it affects that of 

 elongated parallelepipeds, or needles. 



The characters of composition and position, which distinguish 

 the trap rocks are, the cohesive aggregation of their constituent 

 parts, their more or less granular texture, the almost total 

 absence of petrefactions. The structure has this peculiarity, 

 that these rocks are not divided into parallel beds, which pre- 

 serve through a long space the same direction and the same 

 inclination. They occur under the form of prismatic columns, 

 of balls composed of concentric spherical lamina, or of tables, 

 but most frequently in shapeless masses, and at other times in 

 beds, or in veins, which penetrate the rocks of every class of 

 formations, from granites to sandstones and slate clays. The 

 rocks of the trap formation repose on every other, occasionally 

 even on those of alluvion, and are thence the newest of all. 

 They do not form considerable mountains, nor long chains of 



