188 



A third form was when the impression was very superficial, 

 and at the same lime very large. This appeared to be produced 

 by a heavy drop falling perpendicularly on a thin stratum of 

 shale or sandstone, which, being prevented from penetrating, 

 spread itself superficially ; of this two specimens were exhibited, 

 one belonging to the Society, the other to the President. 



A fourth kind of impression was shown in the specimen now 

 sent by Mr. Field. Instead of a rounded impression, the drop 

 produced one of an oblong form, from a heavy shower descend- 

 ing obliquely, and turning up the sand or mud in ridges, lik^ 

 small ripple marks. Such was the fact in the specimen exhibited 

 from Mr. Field. 



. A fifth form, of which many distinct impressions are found, is 

 in relief, caused by the impression being conveyed from the 

 upper surface of one slab to the lower surface of another. 



Some eminent geologists question whether these impressions 

 are the effect of raindrops, or of some other agent. The princi- 

 pal grounds on which such a question could be based, would be 

 the apparent probability of a subsequent shower obliterating the 

 marks of one which preceded, and thus one shower effacing the 

 traces of another. 



This difficulty is not a serious one, however, if we suppose, 

 what was probably the fact, that while the greater number of 

 impressions were effaced, some would have the advantage of a 

 long drought to dry and harden. There is another method of 

 explaining the preservation of these marks, without the suppo- 

 sition of a long-continued drought. Some impressions might, 

 from the heat of the sun, harden sufficiently in a few hours, to 

 prevent the adhesion of newly blown sand, which might thus 

 cover and protect instead of effacing them ; so that when the 

 shales or sandstones were split open, the cast of the rain drop 

 would be retained. Such is no doubt the manner in which many 

 fossil impressions have been preserved. The President re- 

 marked that he could not, however, avoid repeating the observa- 

 tion frequently made, that while the earliest and most labored 

 productions of man have disappeared, the slight impressions of 

 rain drops, ripple marks, and delicate zoophytes have come down 

 to us from a remote period. 



