214 HIDDEN BEAUTIES OF NATURE 



a small cavity lined with crystals of a yellow tint. 

 These are crystals of calc spar, which we must wrap 

 up in paper, as they are easily broken. Their com- 

 position is carbonate of lime. The novice will mis- 

 take them for crystals of quartz or silica. But a 

 trial of the hardness, by means of the tip of the 

 penknife, will show them to be of the softer in- 



Fig. 57. Four Flints, showing Silicified Sponges, known as 'Choanites. 1 

 Natural size. 



gredient. They are very beautiful, and worthy of a 

 place among our other trophies. 



There are four stones, somewhat rounded, which I 

 picked up as we came along. We'll dip them in the 

 sea, so that we can the more readily examine them 

 for traces of animal life. There you see are tubes 



