295 



creek marl, upon the same astringent arglllaceoua sand. It also 

 is of two varieties, one containing only oyster shells and much 

 sand : the other being more clayey, and possessing a larger 

 amount of calcareous matter, and the fossils in a very decayed 

 state. The deposit here has not been much developed, though it 

 obviously deserves to be zealously explored. 



Much of the value of a marl of this description depends upon 

 the condition of the shells which furnish the lime. These may 

 be very indestructible, the case most generally with the oyster ; 

 or on the contrary so easily decomposed, as to be found generally 

 in that chalky and decayed state necessary to constitute a good 

 marl. The largest, and at the same time one of the most 

 abundant fossils in the Slow creek marl, is the Perna tnaxillata, 

 a thick, flat, pearly shell, dividing into numerous thin scales, and 

 valuable from the readiness with which it decays and distributes 

 its calcareous particles. 



In excavating marl in this neighbourhood, it is all important to 

 avoid mixing with the calcareous stratum any of the astringent 

 matter which lies immediately beneath. This is noxious to the 

 crop and seriously detrimental to the soil. It will be known by 

 its taste, and its rarely containing fossils. 



I have accompanied this account with the following analyses 

 of these marls, for the purpose of ascertaining for the inhabitants 

 of the quarter interested, the relative efficacy of different spe- 

 cimens from several localities. 



1. A specimen of average richness from the marl pits of Mr. 

 Elwell, yielded 



Lime, - . . - 25-5 } Carbonate of lime 45*5 

 Carbonic acid, - - 20-0 ) per cent. 



Greenish residue, chiefly clay, 



with some oxide of iron, 54-0 

 Loss, ... - 00-5 



100-0 

 The shells in a very rotten chalky state, principally Pernas, 

 and a good many small turrited univalves. 



