220 Mr. Ii. Mallet on the Origin and Mechanism of 



matic frusta which can be readily detached in crusts or singly 

 from the exterior of the spheroids, are very generally separated 

 by thin interposed surfaces of soft clayey material (kaolin), 

 which has obviously been produced by the decomposition 

 of the felspar, one of the chief constituents of basalts, as is 

 proved by the marked deficiency of alkalies, found by analyses, 

 in the exterior fragments of the spheroids, as compared with 

 the proportion of these in the harder and deeper interior of 

 the lump. From the nature of the contractile forces of cooling, 

 by which the prismatic joints or cheeses of basalt have been pro- 

 duced, as described in the preceding pages, it follows that some 

 residual internal strains due to the final play of the contractile 

 forces, and after those which have produced the prismatic and 

 jointed structure have ceased to act, must exist in everyone of these 

 prismatic segments, producing, in a material so rigid as is basalt, 

 interior planes of strain and of weakness in directions both cir- 

 cumferential and radial (as indicated in fig. 23), and assuming 

 the spheroidal form the more nearly as they approach the centre. 



FiC .Z3 



These planes of strain and of weakness become, under the influ- 

 ence of the chemical action of air and water, of rain and of frost, 

 the planes of separation in which the prismatic segments become 

 broken up, angles and corners disappearing first, until at last 

 an irregular orbicular lump alone remains, or several of these 

 superimposed. And where these lumps are found not regularly 

 superimposed, but thrown together pell mell, as often happens, 

 a careful examination of the directions in which the planes of 

 disintegration are found in the lumps occupying different parts 

 of the mass is sufficient to prove that they have been formed 

 from what were once prismatic segments or cheeses, detached 

 and overthrown by mechanical forces from their pristine colum- 

 nar arrangement. 



Did space allow, much additional evidence might be adduced 

 in proof of the fact that prismatic basalt has not been formed 

 by any process whatsoever from such spheroids, but that, on the 

 contrary, these latter have been produced by decomposition and 

 disintegration of the pointed pieces of basaltic prisms previously 



