﻿510 Rev. T. G. Bonney — Pitchafones and Fehites of Arran. 



traction, and will have a tendency to break up into fragments. The 

 great facility with which spheres are formed under contractile strain 

 will render a perlitic structure a very common result,^ but seeing 

 that obviously space cannot be entirely filled by spherical surfaces, 

 fracture into regular polyhedrons is likely to result, the form of 

 which, in accordance with the principle of least action, will be 

 rhombic dodecahedrons. 



Consider now the case of a single one of these, and suppose it to go 

 on very gradually losing heat uniformly from its exterior, then, just 

 as I have shown happens in the case of spheroidal structure on a larger 

 scale, the sui-faces of equal tension will be approximately spherical. 

 As the mass contracts, all the particles in q.\\j one of these surfaces 

 will be simultaneously acted upon by an equal central force: if then 

 the temperature, pressure, etc., be such that microlithic crj; stalliza- 

 tion is set up, the crystals will grow towards the centre, because both 

 that is the direction of the normal force, and the surface of crystal- 

 lizing temperature is approaching the centre. There is thus a certain 

 analogy with the growth of radiating columns in a spheroidal mass 

 of igneous rock, like the spindle at St. Andrews. The nucleus, when 

 present, is therefore more often likely to be a residuum than an 

 inchaded foreign body. As the mass is in a state of normal strain, 

 it may once or twice rupture along a concentric surface during the 

 formation of the spherule, and this may account for the concentric 

 layers in the spherules. Certainly these are not seldom surfaces of 

 weak cohesion, because the spherule exfoliates easily along them, 

 and they are often covered with a sort of white dust. There is 

 thus a common cause in the development of perlitic and spherulitic 

 structure, namely, a contractile force, but this in the latter case is 

 acting on a crystallizing body. I think, however, that the same 

 explanation will apply to the spherules which do not show a crystal- 

 line structure, and probably to some of the larger felsitic balls in 

 igneous rocks, the outer surface denoting a surface of equal force, 

 which has caused the part within to assume a structure (not neces- 

 sarily crystalline) different from that without. Further, it will be 

 obvious that the process of formation of a line or band of spherules 

 will be analogous to the formation of spheroidal structure in a 

 column of an igneous rock, especially to that case whei'e the spheroids 

 are formed without external rupture of the column. 



Some of the above evidence, such as the banding of pitchstones 

 towards the exterior, and one at least of the banded felsites, also 

 seems to show that this latter structure may be the result of squeez- 

 ing, not of flowing; the same result, as I have pointed out in 

 another case,^ being produced by diverse forces. The former cause 

 is likely to produce the more regular structure ; for it will be readily 

 seen that if a number of particles^ not homogeneous, are mixed 

 together, and then squeezed, so as to be forced to pack into smaller 

 space, there will be a kind of rhythmic arrangement, like getting 



1 See Quart Joum. Geol. Soc. vol. xxxi. p. 152. 



2 loc. cit. p. 15,3. 



