64 Dr. Mac Culloch 07i the Concretionari/ and 



of very different character, and is apparently, in different situa- 

 tions, produced by distinct causes ; circumstances which call for 

 a detail somewhat minute. 



The most remarkable forms of this nature are those which exist 

 in the rocks of the trap family. In this division, these columns 

 are of various sizes, ranging from the diameter of less than an inch 

 to one of many feet; and in height, from a foot, to many hundreds 

 of feet. They are almost invariably associated in groups, so as to 

 occupy the whole, or portions of the stratiform beds occasionally 

 found in the trap rocks. In these cases they are generally parallel, 

 with more or less of exactness ; but, in some, they are variously 

 and irregularly implicated. Occasionally they are even intermixed 

 with amorphous matter of the same nature. They are, commonly, 

 vertical, because the beds which they divide in a perpendicular 

 manner are horizontal ; but they are also occasionally curved. 

 They are often divided by transverse joints of various forms, al- 

 though sometimes simple. The angles of these columns vary in 

 number, but the prevalent forms lie between the four and seven- 

 sided figures : but it is essentially necessary to remark, that the 

 contact is always perfect ; neither vacuity among the angles, nor 

 interval between the approximate sides intervening. 



The more imperfect forms of this description gradually pass into 

 an irregular prismatic structure ; which at length becomes so in- 

 definite as to be confounded with a mere tendency to vertical 

 fracture. 



When these columnar traps are subject to decomposition, it is 

 sometimes observed, that they desquamate in successive crusts, so 

 that a spheroidal nucleus at last remains where there was once a 

 prismatic joint. This has been supposed a proof of a peculiar con- 

 cretionary structure giving rise to the prismatic form, the argu- 

 ments respecting which will be immediately considered. 



As connected with the trap rocks in their general characters, it 

 is proper to observe, that some lavas occasionally assume the same 

 figures. It has sometimes been said that this occurrence took 

 place only where such lavas came into contact with the sea in the 

 course of their progress ; and it has been argued, that a similar 



