514 1\ Crook — The 'Shrinkage' of Septaria. 



latter class. The same procedure, with regard to olivine, is followed 

 to this day in the application of the names tephrite and basanite, 

 nephelinite and nepheline basalt. The distinction between the 

 saturated and the unsaturated minerals is a real one, with important 

 consequences in the chemistry of rock magmas; and petrographers 

 who shut their eyes to a natural distinction of this kind deserve the fate 

 that awaits them at the hands of inventors of arbitrary classiiications. 

 The present paper is an argument for the importance of the above 

 distinction, and a plea for its recognition in petrographic nomenclature. 



VI. — Septaria: a Defence of the 'Shrinkage' view. 

 By T. Crook, A.E.C.Sc (Dublin), F.G.S. 



ACONTRIBrTION to the March number of the Geological 

 Magazine by Dr. A. M. Davies, and another in the August 

 number by Mr. J. E. Todd, support a view formerly held by 

 Professor H. G. Seeley, viz. that the cracks of septaria are due to 

 expansion, during growth, of the outer layer of the nodule. 



For several I'easons this view appears to me to be untenable. It 

 seems to be based on an erroneous conception of what takes place 

 during the growth of a septarian nodule. It exaggerates the signi- 

 ficance of crystallizing force in material deposited from solution. In 

 a rudimentary way a nodule is a crystalline growth, but the manner 

 of tliis growth speaks of a force of crystallization that has been 

 thwarted and not allowed free play. A nodule is of the nature of 

 an imperfectly crystalline precipitate, rather than a robust crystalline 

 growth. It consists of a mass of shapeless microcrystalline granules, 

 each of which was presumably deposited from solution in such a way 

 as to accommodate itself quietly to the surface on which it grew. 

 Surely Mr. Todd makes a mistake when he compares the crystallizing 

 force of material deposited from solution to the expansive force 

 manifested by water in solidifying. There is no analogy between 

 the two processes. 



I think we are justified in asserting that the growth of a septarian 

 nodule in a clay has at least this in common with the growth of 

 a crystal from solution : it presents a sharply defined surface to the 

 medium in which it is growing, and it grows by addition of material 

 to the surface. And if material can be deposited on the faces of 

 a growing crj'stal without the interior being ruptured by the crystal- 

 lizing force when that force displays its full vigour, why should the 

 addition of shapeless and accommodating microcrystalline granules to 

 the surface of a growing nodule result in internal rupture ? 



Moreover, nodules are septarian only when they have incorporated 

 within their substance a considerable amount of colloidal clayey 

 matter. If a nodule consists of fairly pure carbonate, or if it contains 

 sandy matter to the exclusion of clay substance, it does not become 

 septarian. How is this fact explained by the ' expansion ' view ? 



Again, I fail to see how we can allow the last added shell of the 

 nodule to crack the enclosed mass without cracking itself ; and if 

 the outer shell did expand during growth without cracking itself, 



