{^orrenpondence — Mr. J. Monckman. 47/ 



of iron, probably tbe carbonate, was dissolved in the water, and pre- 

 cipitated by the carbonate of lime and magnesia as per-bydrate of iron. 



In order to test the correctness of this explanation, I passed 

 carbonic acid gas through water containing powdered grit, powdered 

 limestone, and a mixture of the two ; and since the stone that was 

 stained was freely exposed to air and light, I was careful that the 

 powders should be exposed in a similar manner. The powders were 

 then allowed to dry and fresh water added. The process was re- 

 peated during several months, but in none of the three cases could 

 I find a trace of iron in the liquid. 



I am led to ask for a publication of my results, in the hope that 

 some of your readers will suggest some other test experiments which 

 may lead to the true explanation of the method by which the stain- 

 ing is produced. James Monckman. 



Yorkshire College of Science, Leeds. 



LLANDOVERY EOCKS IN THE LAKE DISTRICT. 



Sir, — I notice that Mr. Hicks, in his last letter on this subject, 

 is contented to meet his opponents on their own terms, and that 

 he expresses his firm persuasion of the overthrow of their views 

 solely by the evidence afi'orded by the Brachiopods and Trilobites 

 which have been discovered in the disputed beds within the last few 

 months. It may consequently be suspected that the Graptolites 

 which swarm in these beds, but which have been so slightingly 

 alluded to and hurried out of sight, have not a word to say upon 

 the question. I am still, however, inclined to think that a good 

 case can be made out by the aid of the Graptolites alone ; and that 

 the view of the Lower Llandovery age of the Coniston Mudstones, 

 grounded upon this opinion, which I have held for some years, 

 and M^hich was strongly insisted upon in a joint-paper on the 

 Coniston Group by Professor Nicholson and myself, read at the 

 Bristol Meeting of the British Association (1875), needs but little 

 extra confirmation. 



I presume that most palaeontologists are willing to admit that, 

 whether the various genera and species of the Graptolites are of equal 

 systematic value with those of the Brachiopoda and Trilobita or no, 

 they are at least as easily identified by those who have studied them, 

 and that it is tolerably certain that they were correspondingly 

 affected by the conditions which brought about the gradual evo- 

 lution and extinction of the various forms of the latter groups, 

 whose remains are now found upon the same horizons with them. 

 If so, it is surely highly unphilosophical to refuse to allow full 

 value to those points now known with a close approximation to 

 certainty regarding the presence and range of the Graptolites which 

 all would be willing to grant at once, where the other groups 

 are concerned. Following here, therefore, the usual line of argu- 

 ment adopted when the latter are relied upon as indices of the 

 systematic place of their containing beds, we observe — 



Firstly — (a.) The Coniston Mudstones consist of a thin group of 

 shales (black below, and grey or purple above), which reposes upon 

 a limestone containing many of the very highest Bala fossils, and 



