O'Reilly — On the Constitution of Calp Shale. 



567 



tions above given correspond fairly with the analyses of well-determined 

 pumices, as shown in the following analyses : — 







Pumice from Lipari, Obsidian from Telkibanya, 



Klaproth. by Erdmann. 



(Gregg and Letsom.) (Gregg and Letsom.) 



Percentage. Percentage. 



SiOa, . 



77-50, 



74-80 



AI3O3, . 







17-50, 



12-40 



Fe^Os, . 







1-75, 



2-03 



PeO, . 







— 



— 



MnQ, . 







— 



1-31 



CaO, . 







— 



1-96 



MgO, . 







— 



0-90 



Na^O, . 







— 



— 



K3O, . 







— 



6-40 



P3O5, . 



S, . 

 Moisture 

 organic n 







— 



— 



an 

 latt 



er, . 



3-00, 



— 



99-75 



99-80 



Taking the compositions as determined and above mentioned, in 

 connexion with the low densities found, and also with the hardness of 

 the residues which scratched glass after the manner of pumice, it 

 may be allowed that the sum of these characters is sufficient to de- 

 termine these calp residues as being pumices in part. 



In order to substantiate this view tbin sections of the rocks were 

 made and examined. 



Taking the Fingias calp as an example (see PL XI., fig. 1), this 

 under a power of 40 and by ordinary transmitted light showed a light 

 brownish or sepia-coloured ground mass, or matrix, finely granular 

 in places, and presenting porphyritically dispersed therein granules 

 and fragments of what appeared to be fragments of shells or spicules ; 

 with these occur iron pyrites replacing more or less completely the 

 matter of the fragments of shells or spicules. Under polarized light 

 and crossed nicols the fragments showed the presence of lime car- 

 bonate, sometimes recognizable by its cleavages. 



In most cases the alteration of the fossils had reached the point of 

 the complete replacement of the original body, so that the organic 

 structure is either completely obliterated or very imperfectly shown. 

 Under a power of 80, some of these forms are, however, recognizable, 



