344 Lowell. — Geological Facts. 



On the whole, this section presents very interesting and instructive 

 appearances, and such as can be understood only by attributing them 

 to the action of fire. 



Had the whole arrangement been the peaceful effect of water, there 

 is no reason why all the masses should not have been equally strati- 

 fied, nor why any of the strata should have been elevated, contorted, 

 broken and altered. These views derive con%mation from similar 

 appearances which are oljservable in the vicinity, and some of which 

 have been described by Professor Hitchcock in his excellent geology 

 of Massachusetts. We have already mentioned the appearances of the 

 «oapstone quarry in Groton. We may add that on the road be- 

 tween Billerica and Lexington,* and near to the latter town, we ob- 

 served several very perfect trap dykes, intruding among the primary 

 -strata and crossing the road from side to side. 



On. the coast also at Marblehead and Nahant are numerous in- 

 jected veins and dykes which, in connexion with the factsnow men- 

 tioned, and with others detailed by Professor Hitchcock, go to es- 

 tablish the opinion that a vast subterranean igneous agency has been 

 exerted in this region in an early geological era. 



If we return to the rail-road cut at Lowell, we find other very in- 

 teresting and remarkable appearances. 



In that part of the section which is most remote from Lowell, af- 

 ter the cliffs have declined and descended below the soil and the di- 

 luvium, they again rise into view, but not to so great an elevation as 

 in that portion of the cut which has been already described. 



In this place, the section has disclosed granite masses intmding 

 among the strata of mica slate, exactly as the trap does in the other 

 portions of the cut. The granite is perfectly well defined entirely dis- 

 tinct from the mica slate, and stands in vertical walls among its vertical 

 strata ; the granite is, in no case, more than a few feet in thickness ; the 

 minerals of which it is composed (the quartz, felspar and mica) are 

 much larger, and more conspicuous than those of the architectural 

 granite which is every where observed loose or in fixed rocks, all over 

 this region. Gneiss is found in one place interposed between the mica 

 slate and the granite or alternating with the former. From all the cir- 

 cumstances, it appears unphilosophical to assign a different origin to 

 the trap and to the granite in the cases that have been stated. The 

 same causes that produced and injected the one, probably produced and 



* The same that was so memorable in the American Revolution. 



