218 



THE GEOLOGIST. 



most favourable circumstances. Other Trigonise with surfaces precisely- 

 similar are T. elongata, Sow., from the Cornbrash of the same locality, 

 and from the Oxford Clay of Dorsetshire ; T. costata, var. lineolata, Ag., 

 from the grey limestone of Scarborough ; another lunulate and lengthened 

 form from the Upper Trigonia grit, Inferior Oolite near Stroud ; T. moni- 

 lifera, Ag., from the Coral Rag of Weymouth ; and T. marginata, Lye, 

 from the Kimmeridge Clay of Wiltshire. This granulated surface occur- 

 ring, as is now ascertained, in so many species of the costated Trigonise, 

 whose general forms and other characters are very dissimilar, renders it 

 evident that the whole group of the Costatse is characterized by its pre- 

 sence, although we may only expect to discover it occasionally in specimens 

 derived from fine argillaceous deposits, and cleared simply by washing, or 

 by using only a light brush. The other sections of Trigonia having tuber- 

 cles, varices, or serrated ribs upon the sides, appear to have been destitute 

 of this granulated tegument, as are also the recent Trigonise. I would also 

 venture to remark that the value of the granulated tegument as a ground 

 of distinction in the groups of testacea, does not appear to be sufficiently 

 appreciated by some palaeontologists ; that it is of higher value to us than 

 as a separation between species, may be inferred from the fact that in the 

 great family of the fossil Anatinidse it characterizes all the species of the 

 genera in which it occurs, and that the present appears to be the first 

 known instance in which a well-defined genus can be separated into two 

 sections, the one having the surface granulated, the other smooth : in Tri- 

 gonia, however, it is found to pervade only a single but large and well-de- 

 fined group, which in its general characters is as clearly separated from 

 the other fossil groups as from the recent members of the genus. 



John Ltcett, M.D. 



Scarborough, May 4dh, 1864. 



On the Nebular Theory. 



Salford, Mag 17, 1864. 

 Sir, — In No. 75 of your ' Geologist,' I find an article on Planetary 

 Orbits, etc., written with a considerable degree of ingenuity, in which you 

 ask, and I suppose with no objection to a reply, for instances " of the evo- 

 lution of light and heat by sloto condensation of gaseous matter." Chemistry 

 supplies us with, abundance of proof in this respect. One of the most 

 familiar is shale loaded with iron pyrites, which, when exposed to the influ- 

 ence of the atmosphere, often takes fire from the slow absorption of gaseous 

 matter. 



So, on the other hand, excessive heat has greater power than chemical 

 affinity, and will, if supplied in sufficient intensity, release the condensed 

 oxygen again from its compound. Metals, too, have a very great power 

 to condense gaseous matter within their pores, and this power is generally 

 proportionate to their spongy and divided character ; but if heated, their 

 affinity for gaseous bodies is likewise proportionately increased. But as I 

 have just stated, heat has a greater power than chemical affinity, and there- 

 fore no condensation of gaseous matter could take place until it was suffi- 

 ciently cooled to be within the range of chemical power. 



Now the nebular theory assumes that condensation is the result of slow 

 cooling, and could not have taken place in any other manner ; consequently, 

 no universal conflagration and condensation could simultaneously have 

 taken place, as your reasoning supposes. Again, if we closely examine the 

 crust of the earth, especially amongst the igneous rocks, with which we 



