256 



GEOLOGY. 



Fig. 150. 



largely magnified. It is supposed that 

 each of them furnished a nucleus around 

 which the flint, dissolved in the water 

 of the ocean, gathered and solidified. 

 In the upper part of Fig. 157, at a, c, 

 and c?, are shell prisms that are found 

 in flint, those at a being the most com- 

 mon form. They are magnified ten 

 times, as indicated.* The origin of 

 these prisms, which are composed of 

 carbonate of lime, is curious. They 

 come from the shells of certain bi- 

 valves of that period. Now shells are 

 ordinarily made with laminaB or layers, one placed upon 

 another, as you may see in an oyster. But the shells of 

 these strange bivalves are composed of prisms, or many- 

 sided columns packed together, and extending from the 

 inside to the outside. In the figure, at #, we have a 

 piece of flint, with a piece of one of these shells imbed- 

 ded in it. Some of the shell has been dissolved and re- 

 moved, so that you see in the recess the little columns 

 standing up all around, and on the floor of the cavity 

 are the minute depressions in the flint made by their 

 ends. In the specimen of which this figure is a repre- 

 sentation there was a roof of flint over the recess, but 

 this was carefully ground off in order to show the ar- 

 rangement. In the lower part of the figure you see rep- 

 resented a great variety of flinty spicula which come 

 from sponges, and are found in the flint nodules. They 

 are all much magnified, one of them, i, even a hundred 

 times. This is very rarely found. The most common 

 of these spicules are g, h, &, and p. The spindle-shaped 

 one, with raised rings, o, is by no means uncommon. 

 The one resembling it, but having a triradiate summit, 



* Throughout Fig. 157 the character X means multiplied or mag- 

 nified, the numeral annexed showing to what degree the object is 

 magnified. 



