Prismatic Structure in Sandstone, Sfc. 265 



crystalline process. To apply that term to it is incorrect ; it 

 is a concretionary structure, and, from the examples in sand- 

 stone which have been described, it is more likely to have 

 taken place in the solid than in the fluid mass. The same 

 reasoning applies to the prismatic lavas, which, indeed, are 

 themselves sufficient to prove that this structure is the result 

 of heat, at whatsoever period and state of the lava it may have 

 acted. 



Finally, as in the Dunbar sandstone the concentric concre- 

 tionary structure of the prisms has been shown to exist, and 

 that this arrangement is common in the columnar traps, as is 

 proved by their decomposition, it is probable that in the latter, 

 as in the former case, it has been produced during the solid 

 state of the rock. 



It is impossible to illustrate this subject further, in the 

 present state of our knowledge ; nor is it very likely that any 

 satisfactory proofs of it can be derived from nature. It is not 

 easy, at least, to conjecture how such proofs can be discovered. 

 That it might be put to the test of direct experiment I have 

 already suggested ; and repeating the hope that such experi- 

 ments will be instituted, I shall now conclude this commu- 

 nication. 



Experiments on Indigo* 



[From a Calcutta Journal.] 



Chemists in Europe, who have engaged in the examination of 

 indigo, have generally had to deal with the prepared colouring 

 matter as manufactured for the market ; and have therefore 

 limited themselves to the separation and measurement of the 

 foreign ingredients with which it was contaminated, — to the 

 properties of the pure colouring matter itself, — and to the ana- 

 lysis of its chemical composition . The rationale of what passes 

 in the process of the manufacture may be, and has been, deduced 

 with tolerable certainty from the discoveries thus made ; but 

 where we have the whole fermentation carried on among us on 

 an immense scale, — when we have the indigo in its nascent 



