52 Dr. Mac Culloch's Sketch of the 



to those with which they are connected, since they interfere not only 

 with the primary ones, but with almost the latest of the secondary, 

 and are frequently found lying indifferently both above and 

 below these latter. Hence have arisen distinctions which are often 

 merely nominal, and founded on false theoretic views, of traps of 

 one or other age, of first, second, third, and newest formations. 

 The same identical mass will often possess the characters of all these, 

 since it will in one place be found incumbent on the latest, in an- 

 other on the most ancient rocks. Except this, I know not that any 

 certain marks of distinction can be pointed out among the several 

 traps. In the greater number of instances at least which have 

 fallen under my examination, such a relation between an unstra- 

 lified rock, occupying the irregular and imcertain position which 

 trap does, and those regularly stratified ones which maintain a con- 

 stant order of succession, is by no means a criterion by which to 

 judge of its relative order with regard to these. The aid of a diagram 

 is perhaps required to render this statement intelligible to those who 

 have not examined the rocks for themselves.* The same mass of 

 •trap will be found in one place incumbent on clay slate, in another 

 on red sandstone, in a third on shelly limestone, in a fourth on the 

 uppermost secondary strata. In such a case when the separate por- 

 tions are either not all accessible, or when they lie far distant and 

 interruptedly, we might be easily led to conclude that they were sO 

 many distinct deposits, and thus apply to them terms derived from 

 the particular beds with which they were found immediately in 

 contact. A more intimate acquaintance with them, and with the 

 general nature of trap is required to correct these erroneous conclu- 

 sions, and the accompanying* sketch, will explain that which ac- 

 tually occurs in nature, and of which, if I mistake not, instances are 



* See PI. 4. figs. 2 & 3. 



