GEOLOGY. 261 



trap, and in which the sandstone has been deposited. At the junction 

 of the two rocks, where the sandstone lies above, it is extremely difficult 

 to determine where one begins and the other ends, and there is little ap- 

 pearance of metauiorphisin in the sedimentary rock ; but, on the other 

 hand, where the trap is the overlying rock, the line of junction is clear 

 and well defined. The trap in this case is more compact, and the sand- 

 stone, for a distance of three or four feet, is converted into a jaspery 

 mass. These phenomena have been observed at numerous places, both 

 on Isle Royal and Keweenaw Point. The beds of sandstone are not 

 shattered, nor does the igneous rock penetrate them in the form of 

 dykes or ramifying veins. All the phenomena indicate that the 

 igneous rocks were not protruded in the form of dykes between the 

 strata, but that they flowed like lava sheets over the preexisting sur- 

 face, and that the sandstone was deposited on the surface of the 

 igneous mass, in some cases, while the latter was still in an incandes- 



c> 



cent state.' 



INFERENCES DEDUCIBLE FROM THE RAIN-DROP DIPRESSIONS LN THE 

 TRIASSIC AND CARBONIFEROUS ROCKS. 



SIR CHAS. LYELL, in a recent lecture on the impressions of rain and 

 hail in the strata formed in triassic and carboniferous epochs, deduces 

 the following inferences : " These impressions, in rocks of such remote 

 antiquity, confirm the ideas entertained of the humid climate of the 

 carboniferous period, the forests of which, we know, were continuous 

 over areas of hundreds of miles in diameter. The average dimensions 

 of the drops indicate showers of ordinary force ; and show that the at- 

 mosphere corresponded in density, as well as in the varying tempera- 

 ture of its different currents, with that which now invests the globe. 

 The triassic hail-marks, moreover, imply that some regions of the at- 

 mosphere were intensely cold, and, coupled with the foot-prints, worm- 

 tracks, ripple-marks, and the casts of cracks formed by the drying of 

 mud, these impressions of rain clearly point to the existence of sea- 

 beaches, where tides rose and fell, and, therefore, lead us to presume 

 the joint influence of the moon and the sun. Hence we are led on to 

 infer that, at this ancient era, the earth, with its attendant planet, was 

 revolving, as now, round the sun, as the centre of our system, which, 

 probably, belonged then, as now, to one of those countless clusters of 

 stars with which space is filled." 



ROCKS NOT FORMED BY INFUSORIA. 



IT is evident that infusorial animalculee can make their appearance, 

 develop, and multiply, only in those places where they find an abun- 

 dance of the necessary nourishment in a form adapted to assimilation. 

 Several species, and these very widely diffused Infusoria, are distin- 

 guished from other species by possessing certain inorganic constituents, 

 namely, silica, which forms the shells, or cuirasses, as they may be 

 termed, of Naviculce, Bacillaria, &c., and peroxide of iron, which is a 

 constituent of many Gallionellee. The carbonate of lime of the chalk 

 animalculae is precisely similar to the shells of the common molluscous 

 animals. Many persons have pleased themselves with ascribing the 



