Nodules of the Trimmingham Chalk. 385 



pied by chalk of a greyish-white colour, and often crammed 

 with sponge-spicules, many of which are large enough to be 

 visible to the naked eye. The trabecule of flint between the 

 passages are white and porous exteriorly, where they lie in 

 contact with the chalk ; but on breaking them across they are 

 found to consist within of ordinary black flint, with its cha- 

 racteristic greyish spots and patches. 



In the more completely silicified nodules the middle consists 

 of a core of solid flint, and chalk-filled passages exist only on 

 the exterior. These finally disappear in the last stages of 

 silicification ; and the nodule then consists of compact flint 

 throughout. 



The exterior of most of the nodules is covered with a more 

 or less extensive layer of flint, which may form a mere film 

 enclosing the interlaced flint and chalk within, or may attain 

 a thickness of an inch and become continuous with the tra- 

 becula?. of the interior ; on the surface it is even, white, and 

 porous, with no appearance of structure, indeed just like the 

 surface of an ordinary chalk flint. In the completely silicified 

 forms this layer is not present as a distinct structure, being 

 represented merely by the exterior of the nodule. 



To separate the chalk with its spicules from the flint, the 

 nodules were placed in distilled water ; the chalk becoming 

 soft and semifluid, easily fell away from the flint, and with its 

 contained spicules formed a thick sediment at the bottom of 

 the water. To complete the separation, and to remove carbo- 

 nate of lime, sufficient hydrochloric acid was next added: this 

 dissolved the chalk ; and an insoluble residue then remained 

 behind, consisting of silicified coccoliths, Foraminifera, Ento- 

 mostraca, Polyzoa, and echinoderm -spines, siliceous and glau- 

 conitic casts of Foraminifera, and sponge-spicules in great 

 variety. This material was washed, dried, and mounted 

 for microscopic investigation. As a medium for mounting, 

 Canada balsam was, in most cases, found to be unsuited ; it 

 rendered the spicules too transparent for observation ; and so 

 glycerine jelly was substituted for it. 



The flint, after it had shed its associated chalk, was also 

 washed and dried. The parts from which the chalk had 

 been removed showed a white porous surface, sometimes 

 marked by a number of small circular pits, varying in size, 

 many being between -^ and T ~- ff inch in diameter, and re- 

 calling to mind the ostia of a sponge. No definite structure, 

 however, could be detected in this layer. 



Occasionally a broken fragment of a Lithistid or Dictyonine 

 Hexactinellid sponge projects from it ; and so one finds now 

 and then a protruding fragment of a molluscan shell, or the 



