22 



OUTLINES OF GEOLOGY. 



(3.) Joints of injection, which traverse rocks in every pos- 

 sible direction, and are rilled up by rocks or mineral species 

 essentially different from those they traverse. Such joints* 

 when of no great thickness, are called veins, and are the 

 principal source of the useful metals. Joints of injection, 

 when of wide extent and nearly vertical, are known by the 

 name of dykes, and contain only earthy minerals. It is 

 now known that the nuclei of most mountains are only joints 

 of injection of still greater size. 



(4.) Fissures differ from veins in containing no extraneous 

 minerals. The most remarkable of these are found in trap 

 and basaltic rocks, dividing them into prismatic masses. 



(5.) Faults are dislocations of stratified rocks, on the oppo- 

 site sides of which a given layer is found at very different 

 levels. Hence it may be assumed, that it has at this point 

 been suddenly elevated or depressed. Faults may be occu- 

 pied by dykes. A fault is well known in England, in which 

 the level of the strata is changed as much as 900 feet. 



45. The remains or traces of organized matter are said to 

 be fossil. Fossils may occur in the following different states : 



(1.) Relaining their original chemical constitution; 



(2.) Partially altered, and some of their elements recom- 

 bined under the influence of chemical and physical causes ; 



(3.) Wholly changed, so that no more than the form and 

 elements remain, but the latter in new states of chemical 

 combination; 



(4.) Petrified, when the whole of the organized matter has 

 disappeared, and its place is occupied by a mineral whose 

 external figure is precisely that of the organized body ; 



(5.) In impressions, where no other trace is left of the 

 organized body than a mould of its surface in rock. The 

 evidence of the existence of the organized body may be 

 compared to that by which we know a seal has existed when 

 we see its impression on wax, 



