15 



and vegetable, is readily proved. The polished appearance of a 

 stem of wheat is owing to silica ; the leaf blades of some of the 

 coarser grasses are so highly charged with it that they draw blood 

 when passed rapidly between the fingers ; the Equisetce or horse- 

 tails contain so much that a polishing powder is made from their 

 ashes ; the leaf hairs of some plants are wholly siliceous ; while 

 from the intei'ior of one of the bamlioos of India large lumps are 

 sometimes taken in the form of Tabasheer, a more highly refract- 

 ing substance than even the diamond. Of course all this is 

 extracted by the plant from the soil. 



In the animal creation its abundance is more remarkable still. 

 Countless species of the microscopic fom.s known as diatoms con- 

 struct their exquisitely beautiful shells from the silica yielded by 

 the waters in which they live. You can examine some of these for 

 yourselves in the microscopes on the table. Enormous areas of the 

 bed of the Pacific were found by the Challenger to be covered with 

 siliceous, i.e., flinty deposits, consisting chiefly of Radiolaria. Some 

 were brought up by the dredge from a depth of five miles. Mingled 

 with them were the "spicules" of sponges. These are minute 

 needle shaped bodies composed of silica, supporting the soft frame- 

 work of animal matter forming the sponge. Thus silently, and 

 hidden from the eye of man, the processes of nature are carried on 

 in almost abysmal depths, laying the foundations of future worlds 

 and writing in indelible characters the history of their own times. 

 Other sponges anchor themselves to the mud or rocks of the ocean 

 bed by ropes of siliceous fibres, rising up by a single long chimney- 

 like tube. See the Euplectella before you.— Venus' Flower Basket, 

 "without any exception one of the most exquisite of all organic 

 structures known to us." In its living condition this skeleton, 

 which is wholly silica, is completely covered by a thick coating of 

 animal matter called by zoologists sarcode. Another family includes 

 the Hyalonema or glass-rope sponge, which " has the lower portion 

 of its siliceous rope-like axis exactly like a skein of threads of glass, 

 and is sunk in the sand at the bottom of the sea." This is a rarer 

 form judging by the price asked for it at present (two pounds), but 

 you will find an illustration of it, and also of its beautiful spicules, 

 in the volume of Science Gossip on the table. (Vol. for 1872, p. 86) 



Now, remembering all this, and then noticing the very frequent 

 occurrence of sponges and their spicules, shells, and other animal 

 remains in flint, we are led to make a shrewd guess that life in 

 some form or other had to do with its formation. A very large 

 proportion of those flints of curious shape on every wayside heap — 

 semi-globular, spherical, cup-like, branched, &c., will be found to 

 have fossil remains inside. Frequently a dexterous blow of the 

 hammer separates the flint symmetrically, and numbers of spicules, 



