of the Structure of Rocks. 381 



have undergone since their deposition, which is the chief ob- 

 ject of the Professor's paper. 



These changes he classes under two heads, the chemical 

 and mechanical. The former are illustrated by the globular 

 and concretionary structure of sedimentary rocks, such as 

 the nodular flints in chalk, the globular concretions of calc- 

 grit in oolite, the nodules and spheroidal masses in niagne- 

 sian limestone, and the balls of nearly pure quartz in the 

 felspathic slates of Wales. 



These concretions, with the exception of the last, are cer- 

 tainly instances of a peculiar structure, assumed since the de- 

 position of the incoherent materials of which the rocks were 

 formed ; but the Professor admits (at p. 467.) that the felspar 

 slates may have resulted from igneous fusion, and " that they 

 ought, in that case, to be removed into another class, and ar- 

 ranged with the orbicular granite of Corsica, and other con- 

 cretionary trappean rocks." Now, this appears me to be an 

 unnecessary distinction, for the globular concretions of rocks, 

 in formations of all kinds and ages, haveprobably resulted from 

 the same process, viz. the aggregation of their particles during 

 consolidation ; as it would seem " that all that is requisite for 

 such an arrangement is, that the integrant particles of the 

 rocks have the necessary degree of motion among themselves 

 during the lapidification ; but the manner in which this is 

 attained does not appear to be material." 



We are, then, agreed on the fact, that the globular concre- 

 tions of sedimentary rocks are indications of a change which 

 these rocks have experienced since their deposition ; indeed, 

 I have also already stated that " the spheroidal and concen- 

 tric arrangements of the laminae in the sandstone at Dunbar 

 have probably been superinduced since the accumulation of 

 their original incoherent materials." But we widely differ 

 concerning the cause of these peculiar productions. The 

 Professor mentions, over and over again, that the cause is 

 chemical ; whereas I have attributed it to mechanical action, 

 that is, to cohesive attraction. " It appears to be an acknow- 

 ledged principle," he says, " that when different substances in 

 a state of extreme comminution are mechanically mixed 

 together, they have a tendency (o separate and rearrange 

 themselves in masses more nearly homogeneous. The sepa- 

 ration of the pounded flint from the aluminous earth, in the 

 materials prepared for the potteries, has been several times 

 quoted as an instance of this kind of chemical action." 



Surely this cannot be called an instance of chemical action, 

 in which particles of a like nature, placed at sensible distances, 

 attract each other to form a mass. Tiie definitions of chemi- 



