170 Dr. Wallich on the Origin of the 



definite layers in the chalk, the chalk above and below 

 being free both from them and from sponge-spicules. 

 It is difficult to see, in the first place, how a shallow sea came 

 to consist of a strong solution of silica, and still more so to 

 understand how it came to vary in a rhythmical fashion, some- 

 times being concentrated enough to lead to the formation of 

 flints, and again pure enough to leave the intervening chalk 

 almost absolutely devoid of silica." His "statement of fact," 

 as derived from the occurrence of great numbers of sponge- 

 spicules adherent to the nodules, goes for nothing therefore, 

 in so far as the present question is concerned. 



But Mr. Sollas claims to have obtained proof of another 

 kind, in the presence in limestone-rocks of minute quartz- 

 crystals and chalcedonized shells, and occasionally ''numerous 

 grains of silica with a radiate crystalline structure" — and 

 notably in the mountain-limestone of Caldon Low, in which 

 were found a large number of crystals, which he rapturously 

 describes as being " six-sided prisms terminated by six-sided 

 pyramids, the usual form of rock-crystal," and immortalizes by 

 adding that it may " be accepted as a fact that in the moun- 

 tain-limestone these beautiful crystals abound." A great 

 many more details are furnished, relating to the microscopic 

 measurements of these crystals, their being " beautiful polari- 

 scopic objects," &c., all of which information is no doubt 

 excellent in its way, as showing that indubitable, minute, and 

 perfectly-formed rock-crystals have " somehow " been pro- 

 duced from silica in aqueous solution ; but in this, as in the 

 previous case, not a single new or additional fact is brought 

 forward which can in any wise connect the silica of the crystals 

 with the silica of sponge-spicules, or furnish a pretext for 

 assuming that they may not, with just as much probability, 

 have been formed from the silica always held in solution in 

 sea-water, and which is said to be derived principally from 

 the comminuted siliceous ddbris of felspathic rocks brought 

 down to the sea by rivers*. Therefore, until this connect- 

 ing-link between the Trimmingham flints and the spicules 

 found on the chalk adherent to them (but only mechanically) 

 can be positively affirmed, and between " calcitized siliceous 

 sponges and the deposited silica," which, we are told, " is 

 generally to be found somewhere not far off," Mr. Sollas must 

 not be surprised at my regarding these mere " inferences " of 

 his — probable, no doubt, but still mere inferences — with even 



• See ' Student's Elements of Geology,' by Sir Charles Lyell, 1871, 

 p. 265. 



