30-i ANNUAL OF SCIENTIFIC DISCOVERY. 



especially at the centre of gravity of tlie mass, where a nucleus would be 

 formed, and upon whose surface myriads of particles would come crowding 

 inwards and attach themselves ; while, by the general collapse of the entire 

 mass of the nebulous body, resulting, as before said, from the action of gravi- 

 tation of its particles towards its centre of gravity, that heat which was latent 

 in the original or primitive expanded volume of the nebulous mass would 

 come forth and manifest itself as active heat, most intense nearest to where 

 the focus of action lay, where it would result in a glow of fervent intensity, of 

 which we can form no adequate conception. In this manner I conceive the 

 temperature of the nucleus would continue to increase, while the dimen- 

 sions or volume of the nebulous mass went on diminishing, through ages 

 of time, until the temperature of the nucleus reached such a pitch of in- 

 tensity as to begin to check the accelerating influx of particles by the 

 dispersive influence of the intense heat of the nucleus. Then would en- 

 sue an era of retardation in the progressive accumulation of matter upon 

 the nucleus ; and its after history would most probably be governed by 

 the combined action of gravitative accumulation and those changes which 

 would result from the continual escape of the heat of the remaining nebu- 

 lous envelope, and so render the matter of which it was composed more 

 subject to the attractive influence of the globe now existing within it. I 

 conceive that countless ages might thus elapse, through the mutual action 

 of the agencies I have referred to, ere such a globe had commenced the 

 earliest stages of its geological history, which would date from that period 

 when all farther accession of temperature was at an end, and the nucleus 

 (now a planet) began to part with its primitive heat by its radiation into 

 space. Thus I have endeavored to assign as the cause of the primitive in- 

 candescent temperature of planetary masses the action of gravitation 

 upon the nebulous matter of which they are conceived to have been 

 formed : the action of gravitation overtaking in its collapsing influence 

 that gradual decrease of volume which might otherwise have occurred 

 through simple contraction, and so expressing the heat latent in the nebu- 

 lous volume, and causing it to come forth as sensible heat in most active 

 condition, and so manifest itself in a state of intense incandescence in the 

 nucleus or planetary mass. 



ON THE THICKNESS OF THE SILURIAN ROCKS OF GREAT BRITAIN. 



More than twenty years have passed since Sir Roderick Murchison 

 commenced those brilliant researches among the most ancient sedimentary 

 rocks that have secured for him a lasting place among eminent geologists. 

 Before he investigated, analyzed and defined the Silurian system of forma- 

 tions, the knowledge possessed by naturalists of the earliest phenomenon of 

 life in our planet was scanty in the extreme indeed, rather deserving 

 the name of utter ignorance. Under the vague term of " graywacke " were 

 included rocks of different ages, structures, organic characters, and vast 

 thickness. It is difficult, for those whose scientific careers have com- 



