Subordinate Bocks, 17 



appropriate in this essay, which is not so much intended for 

 the correction of geognostical knowledge, as for the applica- 

 tion of that knowledge to a particular object. 



Syenite occurs more rarely in well characterized beds and 

 veins than in irregular masses, but then the relations it exhi- 

 bits are almost the same as those of porphyry, associated with 

 which it is often found, and into which it passes, as it does 

 into granite. Regular beds of this rock, and also well cha- 

 racterized veins, which, as has been already remarked, not 

 unfrequently occm*, are not of special consequence. Exten- 

 sive irregular masses, however, are to be carefully noticed as 

 in the case of porphyry. 



What are usually called traps consist chiefly of two rocks, 

 which, in respect to their component parts, correspond at least 

 so far, that they contain hornblende and, more rarely, augite 

 in preponderating quantity, which determines the colour of the 

 rock. These tw^o are greenstone and hornblende-slate. Some 

 others, as amygdaloid, wacke, greystone, porphyry-slate, ba- 

 salt, &c., of which several are generally included under the 

 trachytes, may here be passed over, as they rarely contain 

 repositories of useful minerals. The greenstone, for the 

 most part, partakes of the features described as belonging to 

 the rocky masses, which occur imbedded inthe slate series ; 

 it is found in veins and beds, but especially in extensive irre- 

 gular masses ; it passes into porphyry, into syenite, into horn- 

 blende-slate, and also into gneiss ; and sometimes it abounds 

 in ores. Hornblende-slate itself agrees with the slate rocks in 

 its geognostical relations, and what has been said here of them 

 applies likewise to it. It passes into gneiss, into mica-slate, 

 and into clay-slate, in the direction of its thickness, as well 

 as of its strike and dip, and the transitions are so similar to 

 those described above, that it would be superfluous to dwell 

 longer upon them. In some districts, both hornblende slate 

 and greenstone have proved rich in metalliferous contents, 

 and they ought, therefore, on no account to be passed over 

 unnoticed. 



Limestone is a mountain rock of especial importance. It 

 occurs partly in the central granitic mass, especially when the 

 latter possesses a tabulai* structure, or, according to the usual 



VOL. XXIX. NO. LVn. ^JULY 1840. » 



