CORRECT DEFINITIONS OF MARL. 373 



cleared away but little of the before general obscuration on this 

 subject in their own country. Of such remaining ignorance, or its 

 appearance, striking and recent examples will be presented. 



The passages to be quoted will exhibit so fully the contradictions 

 and ignorance generally prevailing as to the nature of whatever 

 was called marl, and the operation of calcareous manures generally, 

 that it will not be required for me to express dissent in every case, 

 or to point out the errors f facts or of reasoning, which will appear 

 so manifestly and abundantly in some of the quotations. 



But besides the errors and even absurdities of opinions and prac- 

 tices in regard to marl or lime, which some of these passages will 

 show, there will be presented in connexion some correct, precise, 

 and very interesting facts. Among these will be definitions and 

 descriptions by recent authors of marl proper, and also the varieties 

 known in Britain by the provincial names of " clay" or " clay 

 marl," and the "shell marl" formed only in ancient lakes, since 

 changed to peat bogs. These passages, though some of them are 

 the very latest in the order of time, will be offered first so that 

 what is sound and true may be kept in view, through all the mass 

 of error that will be afterwards presented. 



1. "Compact limestone, by an increase of argillaceous matter, passes 

 into marl." " Marl is essentially composed of carbonate of lime and clay, 

 in various proportions." CleavelancTs Mineralogy. 



2. " Marl is a compound of carbonate of lime, argil [finest clay] and of 

 silicious sand. The sand appears to be only in a state of mixture, and 

 may be, when not very fine, separated easily. But the argil and carbo- 

 nate of lime in marl (like the alumina and silica in argil), seem to be a 

 [chemical] combination, and not a simple mixture." Puvis Essai sur la 

 Marne. 



"Marl seems to be, in most cases, a formation of fresh water." Puvis 

 Translation, Farmers' Register, vol. iii. p. 692. 



3. " Marl is a combination of carbonate of lime and clay. These two 

 bodies are usually found in so complete a state of amalgamation, that it is 

 impossible to distinguish the particles of one from those of the other, either 

 with the naked eye, or with the aid of the microscope." " When water is 

 poured upon marl, that fluid penetrates, with greater or less facility, into 

 all its pores, destroys the cohesion of the parts, separates them from one 

 -another, and reduces them to a fine powder. This is one of the essential 

 properties, which serves as the first distinction of marl," &c. 



"It certainly cannot be admitted as a principle that any kind of earth- 

 which loses its aggregation in water must necessarily be marl, since some 

 very poor clays are affected in the same manner ; but if any kind of earth 

 is not spontaneously reduced to powder by the action of water, we may 

 feel convinced that it is not marl. Every kind of marl, even that which is 

 called 'stony,' becomes soft and. pulverized in water." <Von Thaer's Prin~ 

 dples of Agriculture, 



It appears from different authors that the proportions of carbo- 

 nate of lime in marl usually vary from 20 to more than 60 per 



