352 Notices respecting New Books : — ^Darwin on the 



No. 6. 



Opake brown sphserulites, drawn on an etilarged scale. The upper ones are externally marked 

 wit.i parallel ridges. The internal radiating structure of the lower ones is much too plainly 

 represented. 



united together, intersects, as represented in the woodcut, No. /, a lajer of 

 similar composition ; and after running for a short space in a slightly curved 

 line, agjiin intersects it, and likewise a second layer lying a little way be- 

 neath that first intersected. The small nodules also of obsidian are some- 



No. 7. 



A layer formed by the union of minute brown sphferulites, intersecting two othersimilar layers ; 

 the whole represented of nearly the natural size. 



times externally marked with ridges and furrows, parallel to the lamination 

 of the mass, but always less plainly than the sphaerulites. 'J'hese obsidian 

 nodules are generally angular, with their edges blunted ; they are often im- 

 pressed with the form of the adjoining sphaerulites, than which they are 

 always larger; the separate nodules seldom appear to have drawn each 

 other out by exerting a mutual attractive force. Had I not found in some 

 cases a distinct centre of attraction in these nodules of obsidian, I should 

 have been led to have considered them as residuary matter, left during the 

 formation of the pearlstone, in which they are imbedded, and of the sphae- 

 rulitic globules. 



"The sphaerulites and the little nodules of obsidian in these rocks so closely 

 resemble in general form and structure concretions in sedimentary deposits, 

 that one is at once tempted to attribute to them an analogous origin. They 

 resemble ordinary concretions in the following respects, — in their external 

 form, — in the union of two or three, or of several, into an irregular mass, or 

 into an even-sided layer, — in the occasional intersection of one such layer 

 by another, as in the ease of chalk-flints, — in the presence of two or three 

 kinds of nodules, often close together, in the same basis, — in their fibrous, 

 radiating structure, with occasional hollows in their centres, — in the co- 

 existence of a laminary, concretionary, and radiating structure, as is so well 

 developed in the concretions of magnesian limestone described by Professor 

 Sedgwick*. Concretions in sedimentary deposits, it is known, are due to 



* *' Geological Transactions, vol. iii. part i. p. 37." 



