382 
My observations on the sandstone series of rocks have 
been so few, that beyond an experiment which will be 
presently described, I have little evidence to adduce. I 
prefer, therefore, advancing at once to a statement of my 
experiments on the formation of the laminated rocks, such as 
clay slates. 
The modes of experiment were as follow : — 
Between a zinc and copper plate properly connected in a 
vessel prepared for the purpose, was placed a mass of clay 
in a very soft state, a condition, in fact, of thin mud. The 
electric current was developed commonly by the action of 
ordinary water on the zinc plate, to which sometimes a small 
quantity of sulphuric acid was added. 
On other occasions, a voltaic battery of tolerable constancy 
was employed, and the clay was placed between plates of 
platina in another vessel, under the latter form of the 
experiment the results were never so decided as in the pre- 
vious one ; but the indications were, in both instances, of 
the same order. On one side, the mass of clay when allowed 
to dry between the plates, (the action of the current being 
maintained for several weeks, sometimes months,) was found 
to be indurated in a remarkable manner ; and on the other 
laminated. Figure 3 represents the structural arrangement 
of masses of clay thus treated. 
On one side distinct lamination had taken place, the clay 
breaking off in thin shales, and the depth to which this pro- 
ceeded was always determined by the length of time the clay 
was under voltaic influence. On the other side, considerable 
induration was invariably produced, and this hardening was 
found to be due to the formation of small concretions, 
arranging themselves along tolerably well-defined curve lines. 
The experiment, in connexion with the sandstone rocks, 
was one in which a mass of the Bath oolite was substituted 
for clay. The stone was kept under experiment for many 
