634 Geological Societtj : — 



appear in the orj'stallizing glass. ' Porous spherulitcs ' are once 

 more mentioned, in order to call attention to the ' feather-like ' 

 crystals which often distinguish them, and of which an explanation 

 is given in Part II. Reference is made to the conditions which 

 favoured primary devitrification at Obsidian Cliff; and the author, 

 leaving general principles to be discussed in the second part, men- 

 tions one or two special types of primary devitrification. These 

 are concerned with the probable formation of eutcctic zones, or 

 patches ; either following the crystallization of an overplus of any- 

 given material, or as a residuum. After a brief reference to 

 secondary devitrification, this part of the paper concludes with a 

 summary in which the several relations of secondary to primary 

 devitrification-structures are given. 



Part II.— By Prof. T. G. Bonney. 



Crj'stallization in a colloid mass involves an orientation and 

 commonly a separation of the molecules ; a process illustrated in an 

 early stage by the formation of microliths in a glass, and the 

 devitrification of the latter when it is heated without actual melting, 

 or \}j a metal becoming crystalline under strains. Certain condi- 

 tions, such as slow cooling, supersaturation, and the presence of 

 inclusions — anything causing discontinuity — are favourable to crys- 

 tallization, some special cases of which are discussed in the paper. 

 The structures thus formed in rocks may he classified as (1) the 

 linear, and (2) the granular ; and the former may be subdivided 

 into {a) the rectilinear, (b) the curvilinear. Spherulitic structure 

 in its simpler form falls under {a), and is at first little more than a 

 radial grouping of molecules, the process becoming, as described, 

 gradually more complicated. Of this, ' graphic ' or ' pegmatitic ' 

 structure is a final stage, where two minerals are crystallizing out 

 of a solution, and one has slightly the advantage over the other, so 

 that it virtually forms a skeleton-crystal. Into this the ordinary 

 radial growth of a spherulite may be seen to pass ; likewise also 

 examples of (a) into those of (6) : the latter being due to the ' leading ' 

 mineral meeting with a rather stronger resistance, as if a crystal were 

 forming in a ver}' tough jelly. An experiment of Messrs. J. I' Anson 

 <t E. A. Pankhurst (Min. Mag. vol. v, 1884, p. 34) on the formation 

 of tubes of colloid silica from a fluid alkaline silicate, affords a good 

 illustration of this curvilinear growth. Eesistances, as the author 

 has pointed out in an earlier paper, are favourable to actinolitic and 

 branching growths, and the various types of structure mentioned 

 above can be shown to be dependent on them. 



The granular structure is next discussed, and explanations are offered 

 of its varieties. This, on a microscopic scale, is often a result of 

 devitrification, where (so far as is known) there has been no marked 

 rise of temperature ; and the author shows how this is affected by 

 greater or less freedom of molecular motion, discussing also cases 

 in which a crystalline mass, like a spherulite, has undergone a later 

 re-arrangement. 



