420 



Scientific Geology. 



ly : since this might have resulted from two paroxysmal efforts of the 

 same eruptive force. 



The greenstone occupying these veins at Nahant, varies in texture 

 from the finest siliceous slate, to coarse sienite. The veins sometimes 

 run parallel to the strata, and would be regarded by' some geologists 

 as regularly interstratified with the slate. And they would be con- 

 firmed in this opinion by the apparently distinct stratification of one of 

 the varieties of the greenstone on this promontory, particularly at a 

 place about fifty rods northwest of the large hotel. The rock here is 

 coarse and appears to be passing into sienite: It is divided into paral- 

 lel portions by seams a few inches apart ; and looking only to this 

 spot, I do not see why the evidence of stratification is not almost com- 

 plete. But if, as an almost universal fact, greenstone, sienite, and 

 granite, are certainly not stratified, it is a presumptive evidence that 

 they never are so : Apparent exceptions it is reasonable to explain 

 on other principles. And in the present case there is a principle that 

 may afford a solution of such a case as that mentioned above. I refer 

 to the formation of concretions in the unstratified rocks. That they 

 are frequently formed in all the varieties of these rocks, the records of 

 geology will testify. Usually, however, they are only a few feet, or 

 a few inches in diameter. But what reason can be adduced why they 

 may not be produced of mountainous bulk ? Their origin is, indeed, ob- 

 scure : but probably their formation depends upon some modification 

 of the laws of crystalization. And if so, who can tell through how 

 large a mass of matter these laws may operate. In an exarrfple of ap- 

 parent stratification in our granite, which I shall shortly describe, we 

 have an opportunity of ascertaining that the layers are of a spheroidal 

 form, although they cover a mountain of no inconsiderable size. And 

 in all cases which I have met with, it is only a part of the rock that 

 is apparently stratified. This is the case at Nahant. There must 

 then have been a peculiarity in the cause that could thus have affect- 

 ed one portion of the formation and not another. In some instances I 

 have explained a partial and non-continuous stratification in rocks, 

 (ex. gr. limestone and hornblende slate,) by supposing one part en- 

 entirely, and the other only partially melted. But in the rocks under 

 consideration, the division of the pseudo-strata is too distinct to admit 

 such an explanation : while the schistose structure is always wanting. 

 Upon the whole, it seems to me that in the present state of our knowl- 

 edge, sound philosophy requires that apparent stratification in rocks 



