IPS .SEPTARIA.W CONCRETIONS IN CLAYS. 



The Chiastolite Slate, on the flanks of Skiddaw, abounds in cry- 

 stals of chiastolite and hornblende, which have been developed in situ, 

 and the development of these and other minerals must always be con- 

 sidered to depend chiefly upon the original composition of the clay ; 

 for even the augite, in the consolidated peperino from Vesuvius, has 

 become changed into a zeolitic substance, which has thoroughly hard- 

 ened the whole deposit. 



The chiastolite in some of the black slate of Ivy Bridge in Devon- 

 shire may be seen in process of being changed into mica, and masses 

 of mica are otherwise developed about special centres. The slate of 

 Liskeard contains scattered concretions, composed of crystals of mica 

 and quartz, which have been formed in situ. These little masses, only 

 T ^th of an inch in diameter, have been termed the very germs of 

 mica schist, and the rock appears to present a first stage of the deve- 

 lopment of that material. In other slates the concretions of quartz 

 and mica have become so abundant as to have coalesced and thrown 

 the residue of the rock into patches, which may possibly be incipient 

 garnets. This slate is seen near to granite near Wicca Pool. From such 

 a rock the transition is easy into mica schist, where the crystals all 

 lie in definite directions, which may be parallel to the plane of strati- 

 fication, parallel to the plane of cleavage, or parallel to joints; but 

 even when the structure is perfectly developed, original grains of sand, 

 milk-white, water-worn, and angular, often occur, associated with fel- 

 spar a good deal decomposed, and felspathic grains, which appear to 

 have been derived from a felsite. It has been estimated that these 

 grains of sand occur in about one-fifth of the slates and one-fifth of the 

 schists. The grains of quartz often contain crystals of rutile and 

 minute granules and fluid cavities, in which sometimes crystals of 

 alkaline chlorides are found. No granite or felsite is known in which 

 the quartz grains show all the characters exhibited by the schists of 

 the central Highlands of Scotland, and hence the parent rock for those 

 ancient muds is supposed to have been either a quartz felsite, or a 

 granite and a felsite denuded together. The proof that the crystals 

 of schists were not deposited where they occur is furnished by the 

 fact that they are fitted and dovetailed together in a complicated and 

 accurate manner, such as always occurs when crystallisation takes 

 place in situ. 



Septaria. 



Septaria occur in flattened ovoid masses in nearly all clays. They 

 usually contain 65 per cent, of carbonate of lime, 18 per cent, of silica, 

 and the remainder is about equal proportions of alumina and protoxide 

 of iron, though the amounts of these ingredients vary. They are often 

 called cement stones, because used for the manufacture of roman 

 or hydraulic cement. The stone is not durable, though it is dense and 

 hard. When these concretions are broken across, they are observed 

 to show numerous cracks, which are widest in the middle of the con- 

 cretion, and radiate in many directions from the centre towards the 

 circumference. When these cracks are filled with crystalline deposits, 



