Cole — Origin of the Orbicular Granite of Mullaghderg. 155 



1904, when he stated that the distance between the centres of "orbules" 

 may be determined by the rather short distances through which osmotic 

 diffusion operates in limited time. The radial structure in the spherulites of 

 the rock described by Lawson depends on the grouping of olivine, which 

 controls the associated felspar ; and Lawson observes that this arrangement 

 may be due to radial movement of the diffusion currents. No definite sugges- 

 tion is made, however, as to the nature of the bodies from which the diffusion 

 took place. Diffusion implies the presence of something capable of spreading 

 through a medium. The spacing of the centres must surely be due, not to 

 the distances reached by the solute, but to the original distribution of crystal- 

 segregations or inclusions in the gabbro magma. Seeing that the centre of 

 the spherulites now consists of a granular basic felspar, probably anorthite, 

 with olivine and a little hypersthene, it is of interest to enquire whether a 

 concentration of calcium and a loss of iron and magnesium has not resulted 

 from the process of diffusion. 



Johnston-Lavis in 1894 postulated the presence of a liquid, which might 

 consist of molten silicates, as the agent of diffusion, and as a necessary factor 

 in segregation. In his still more important paper published in collaboration 

 with J. W. Gregory" later in the same year, it is shown how a banded or 

 zonal structure may be a consequence of diffusion. Nothing can be clearer 

 than the few lines in which the authors anticipate, in explaining the layers 

 of olivine in limestone, the principle so admirably developed and illustrated 

 experimentally by Raphael E. Liesegang 43 in recent years. 



Von Ohrustschoff," as far back as 1891, realised that the absorption of 

 inclusions in igneous rocks implies a diffusion of matter in opposite directions. 

 He attributes the radial structure at Ghistorrai, Sardinia, to the slowness 

 with which the magma was able " von aussen nachzudringen und nach aussen 

 zu diffundiren." Johnston-Lavis 16 held that diffusion was responsible for the 

 development of concretions of calcium earbonate in marl. Liesegang 46 states, 

 somewhat more definitely, that in the case of such concretions, " Da der Kalk 

 von auszen kommt, musz etwas, was den Kalk fallt, von den Zentren 



43 " Eozoonal Structure of the Ejected Blocks of Monte Somma," Sci. Trans. R. Dublin 

 Soc, vol. v (1894), p. 264. 



43 " Uber die Schichtungen bei Diffusionen" (Leipzig, 1907), and especially "Geologische 

 Diffusionen " (Dresden, 1913). 



44 Op. cit. (5), p. 129. 



45 Op. cit. (39), p. 137. 



40 " Geol. Diffusionen," pp. 159 and 171. S. Taber, " The Growth of Crystals under 

 External Pressure," Amer. Journ. Sci., vol. xli (1916), p. 555, points out how concre- 

 tions, such as flints in limestone, may replace the pre-existing rock by exerting pressure 

 during crystal growth, and hence promoting the solution of the material where this is 

 more soluble under pressure. 



