172 DIORITE, TRAP ROCKS, &c. 



obsidian and of pe'rlite, with all their gradations to pumice. Their 

 abundance and character vary according to locality ; they prepon- 

 derate in some countries, while in others scarce a trace of thorn is 

 to be seen. 



14. Di'orite, trap rocks, amygdaloid, <^c. There is nothing 

 more analogous to basalt than certain black rocks, some of which, 

 according to the numerous gradations they present in deposits in 

 which the elements are distinct, must be mixtures of albi'te and of 

 amphibole, and others are of an unknown, or at least doubtfu 

 nature. The first are designated in France under the name of 

 di'orite, and in Germany they are known as gmnstein. The 

 others have long borne the appellation of trap (from the Swedish, 

 trappa, a stair), the nature of which it is still impossible to deter- 

 mine definitely. These rocks bear some relation, as much by 

 their position in certain localities as by their mineralogical charac- 

 ter, to certain substances called amyg'dalo'idti, in consequence of 

 the nodules of various matters they contain, which are known in 

 England as loadstone, and whinstone, the nature of which is often 

 not better known. 



15. For a long time these rocks were supposed to be of aqueous 

 origin ; but it is now ascertained that they are from igneous 

 causes. 



10. At first, in spite of the absence of scoria'ceous matters, 

 these rocks, and especially those named trap, present all the 

 features of basaltic deposits ; they are found in isolated hillocks, 

 or in tables of greater or less extent ; their mass is often divided 

 into prismatic columns, which possess precisely the same appear- 

 ance as basa'ltic colonnades, giants' causeways, and all the forms 

 of basa'lt. On the other hand, these substances are frequently 

 found in veins ; and it is remarked that these veins or seams ter- 

 minate above in a pointed mass 

 (a, Jig. 282), or in their course send 

 off small ramifications (6) into the 

 rocks through which they pass 

 small masses (c), sometimes isolated, 

 sometimes communicating with the 

 principal mass by a thin seam. The 

 enclosing rocks are sometimes occa- 

 Fig. 282.- Veins of trap -Iceland. siona j ]y perforated by small ramifi- 

 cations, and even to the finest fissures. These circumstances evi- 

 dently show these are not cracks filled from above, and can be 

 regarded only as injections from the interior, thrown with sufficient 

 force to penetrate the smallest fissures, to detach and carry away 

 fragments of rock sometimes found in their substance, as at d. 

 17. All these circumstances are exactly the same as those seen 



14. What is di'orite ? What is trap ? 



15. What is the origin of di'orite and trap ? 



16. What are the characters of trap ? In what form is trap met with ? 



