CoLK — The Picture-Roclc or S'rn'hrd Ror/r nr/ir HdlJimiil/nii. 1 lo 



The division of an iiiLnisive slieot into rectangular rather than hexagonal 

 columns has been noted by Jlr. J. Volney Lewis' in the ophitic dolerites of 

 the Palisades along the Hudson Elver. Mi-. Lewis also describes a 

 spheroi<lal system of joints occurring within the rectangular columns. The 

 main joints in the ricture-Eoek might have been attributed to torsion, were 

 it not tliat they are clearly contemporaneous with the onion-like structure. 

 The intrusive rhyolite of Tardree Mountain, in the county of Antrim, is 

 similarly divided into sheet-like masses, rather than into columns, by a 

 system of predominant joints, the cross-jointing being at right angles to 

 these, and giving the effect of square columns in places. Probably such 

 rectangular jointing may be more common than is generally recognised 

 in intrusive sheets and sills, the edges of which appear columnar when 

 exposed. 



Where spheroidal jointing has also taken place, decay usually goes on 

 inwards from the main rectilinear joints ; and the spheroidal features become 

 more and more manifest, as limonitic crusts are formed over the successive 

 curving surfaces. In time a crumbling clayey material, full of detached 

 crystals, separates the spheroids, and the residual cores of the latter ultimately 

 lie loosely in a sort of loam. The abrupt contrast between the decayed outer 

 layers and the unaltered central core has led in some cases to the suggestion 

 that such spheroids are A'olcauic bombs thrown out into a bed of ash.- 



We must bear this contrast in mind when we attempt to explain the 

 features of the Picture-Eock. The upstanding spheroids, connected with the 

 main mass by their back surfaces only, are of various sizes, and of somewhat 

 irregular form. Some are distinctly flattened from one side to another ; and 

 the shape of each one depends on the proportions of the compartment in 

 which it has arisen. A large spheroid measures 20 cm. in its longest 

 diameter. The hollow round the spheroids may extend 6 cm. deep into the 

 rock. In se\'eral cases a spheroid has fallen out altogether, lea\ing a mere 

 empty box-like compartment. It is clear that the soft loam resulting from 

 the decay of the outer shells has been completely washed away. The growth 

 of lichens over the residual cores seems to show, on the other hand, that 

 decay is now slow, if not arrested. I should regard this as due in some eases 

 to the absence of furtiier curving joint-surfaces. The unjoiuted core has 

 been reached by the removal of all the outer layers, except at the back of the 

 spheroid, where rain penetrates with dithculty. Such spheroids will now 



' " I'etiograpliy of the Newark Igneous Roiks," Geol. Survey of New Jersey, Ann. Report, 

 1908, p. 107. 



- See references iu (i. Cole, " The Red Zone ia the Basaltic Series of the county of .Vnliini." 

 Geol. Mag., 1908, p. 341. 



