6S 



Ascension. 



PAET I. 



spkaerulites with the parallel ridges and furrows are 



represented on an enlarged scale, but they are not well 

 executed : and in the lower part, their usual manner of 

 grouping is shown. In another specimen, a thin layer 

 formed of the brown sphaerulites closely united together, 

 intersects, as represented in the woodcut, No. 7 a layer 



No. 7. 



A Layer formed by the union of minnte brown sphasrulites, intersecting two other 

 similar layers : the whole represented of nearly the natural size. 



of similar composition ; and after running for a short 

 space in a slightly curved line, again intersects it, and 

 likewise a second layer lying a little way beneath that 

 first intersected. The small nodules also of obsidian are 

 sometimes externally marked with ridges and furrows, 

 parallel to the lamination of the mass, but always less 

 plainly than the sphasrulites. These obsidian nodules 

 are generally angular, with their edges blunted : thev 

 are often impressed with the form of the adjoining 

 spha?rulites. 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 residuarv matter, left during 

 the formation of the pearlstone, in which they are 

 embedded, and of the spha?rulitic globules. 



The sphrerulites and the little nodules of obsidian 

 in these rocks so closely resemble, in general form and 



