46 Correspondence — Dr. Charles Riclietts. 



majority of cases to pay the travelling expenses of the referee. I 

 am aware that disputes cannot always be settled in this way ; but 

 the three sections I have named are so clear and simple, that a third 

 party can hardly fail to come to an immediate decision, and we 

 should select a person whose award would carry weight. If Mr. 

 Blake refuses this challenge, I will offer to take any competent 

 geologist to the sections, and to forfeit five gunieas to a hospital if I 

 fail to convince him. Ch. Callaway. 



November 20th, 1891. 



" CONCRETIONS " IN MAGNESIAN LIMESTONES. 



Sir, — There is another possible method, besides those suggested 

 by Messrs. Garwood and Jukes-Browne, by which the globular and 

 pseudo-coralline and other forms so remarkable in the Magnesian 

 Limestone of Durham may have originated — the mechanical ; and 

 this is slightly alluded to by Professor Sedgwick (page 92). 



Many years ago a friend presented me with a considerable series 

 of specimens obtained from the neighbourhood of Sunderland ; their 

 examination seemed to indicate that their forms were due to mecha- 

 nical action, but it was diflScult to imagine how the principle on 

 which a school-boy's marbles, " alleys, tors and commoners," are 

 formed from cubical fragments of stone, could have been applied 

 by natural means ; nor did a visit to the extensive quarries at Fulwell, 

 north of Sunderland, assist in solving the problem. The promontory 

 on which Tynemouth Castle stands may possibly help to afford a 

 solution. The strata forming the base of the cliff consist of Coal- 

 measure Sandstone, on which rests a bed composed of angular frag- 

 ments derived from the same, and over this the Permian Limestone 

 has been deposited. This Limestone is full of cavities resembling 

 in appearance some examples of vesicular trap. 



I would suggest that during the formation of the limestone, and 

 whilst it was still in a more or less plastic state, gases evolved from 

 the decomposition of vegetable matter forming beds of Coal made 

 their way through the basement beds of limestone. Such vesicles 

 as occur at Tynemouth might be expected to have been formed under 

 these conditions, and it is quite possible that the globular forms 

 of the so-called " concretions," which occur near Sunderland, may 

 have originated from a similar cause, though under somewhat 

 different circumstances, such as the amount of gas evolved ; the 

 amount to which the lately deposited limestone had consolidated, etc. 

 Whatever was the cause of the fashioning of the globular masses, 

 the same must have been the instrument by which the coral-like 

 forms were shaped. 



I much regret being so circumstanced that I could not conveni- 

 ently carry out a series of experiments to determine the possibility 

 of the globular forms being due to the cause suggested. The only 

 rough attempt made was so far satisfactory that the passage of 

 carbonic acid, generated beneath clay in a plastic state, resulted in 

 the production of many small rounded forms. 



Birkenhead, November, 1891. Charles KicKBTTS. 



