OVERLYING AND TRAP ROCKS. 105 



granites and porphyries are so distinct and so complex 

 as we find them, considering their community of ori- 

 gin in all cases, and of date in many. Yet the ori- 

 ginal materials may constitute the fundamental diffe- 

 rence ; as it may also he, what the result is, the supe- 

 rior quantity of quartz. If, in concluding, I have 

 adopted the un-English term u overlying," it was for 

 want of a hetter. The term trap cannot conveniently 

 he so far generalized at present ; and to use that of 

 porphyry exclusively for the antient ones, is to call 

 that porphyry which is not so, and also to exclude 

 the later porphyries of the trap family. The porphy- 

 ritic character is no distinction, and can only tend to 

 perpetuate confusion, like Syenite; while the tendency 

 to suhstitute system for fact must he discouraged in 

 every manner. 



The substances thus associated under the term of 

 Overlying rocks and Trap, are recognised, first, by 

 their posteriority to many, if not to all of the primary, 

 and similarly to some, or all, of the secondary strata : 

 secondly,, by the prevailing, if not universal absence 

 of the stratified disposition arid the mechanical tex- 

 ture; the whole, with the exception of certain conglo- 

 merates, being of a crystalline or a chemical nature : 

 and thirdly, by intruding, in the form of large masses 

 or veins, among all the neighbouring rocks, while 

 these intrusions are also frequently attended by 

 changes in the invaded substances. The terms for- 

 merly applied to the several members of this family 

 were neither just, nor precisely used, nor sufficiently 

 numerous. Unwilling to change where it could be 

 avoided, I have defined and limited those which I 

 have adopted, while I have, added such as seemed 

 necessary. Two are entirely new : and necessarily 

 so ; because these most important rocks had either 



