356 SKY. GEOLOGY. SILICEOUS SCHIST. 



It may be said generally, that the blackest varieties 

 are the hardest, and approach most nearly in their 

 fracture and lustre to flint, although there are some 

 exceptions to this rule. These are also the most brittle, 

 and they are often indeed so fragile as to fly into small 

 pieces before the hammer, rendering it difficult after 

 numerous trials to procure a single specimen. It is in 

 this state that the siliceous schist is known by the name 

 of Lydian stone. While some of the specimens have 

 the lustre of flint, others present a dry earthy surface, 

 and with this aspect they commonly offer greater resistance 

 to the hammer. The fracture is in all cases more or 

 less conchoidal, but is largest in those specimens that 

 present a dull surface ; in the most flinty varieties a 

 conchoidal and angular fracture are combined. Some- 

 times the fragments are found united to limestone in 

 a parallel manner, clearly proving their original connexion; 

 and in these cases the limestone, instead of the usual 

 earthy aspect, presents a crystalline texture. 



The fragments of chert are equally abundant, but 

 being of a more durable nature are found of a larger 

 size. They vary in colour from a greyish white to 

 blueish grey ; sometimes they are mottled, and occa- 

 sionally they exhibit tints of greyish green. The fracture 

 is commonly plain, or very large conchoidal, but so dif- 

 ficult to produce, that a hammer of four pounds weight 

 is often insufficient to detach a single specimen from 

 the block. Now and then they are somewhat more 

 brittle, but in every case the thin fragment when pro- 

 cured has the sharpness of glass with a toughness ap- 

 proaching to that of iron. This chert strongly resembles 

 that which is entangled in the pitchstone of Egg; still 

 more nearly that of the Shiant isles, which will hereafter 

 be shown to originate in the same substances, namely, the 

 different beds of the lias limestone. 



That evidence which is here wanting with regard to 

 the natural position and affinities of the chert and the 



