414 



THE AZOIC SYSTKM AND ITS SUBDIYISIONS. 



dense bad been found in tbo serpentine (bmestonc) quarry at "Devil's 

 Den," Newbury. Prof. Hyatt then remarked that "the rocks of this 

 county [Essex] had been bypotbetically referred to the lowest known 

 series of Laurentian strata"; but that this was the first instance "in 

 which any positive evidence has bocTi produced of their actual age." 

 (Bull. Essex Institute, 1869, I. pp. 141, 142; American Kat., 18G9, 



III, pp. 498, 499.) 



In 18G9, Prof N. S. Shalcr remarked : 



" There can be no doiibt that the syenites, wliich make up so large a part of 

 the exposod rucks of Eastern Massachusetts, are the oldest materials found in 



this rei-'ion The most remarkaldc fact which has come ujidcr my ub- 



servati(ju is the existence of x:)lanes of separation in this syenite, Avhich cannot 

 be referred to joints That I am nut mistaken in referrin^^ these frac- 

 tures to bedding, is, I believe, abundantly proven by the details of structure of 

 tlie syenite itself, as well as by the relatiuns it bears to the un(|ucslionably 



straLilied rocks which rest upon it There are visil)le on the surface of 



considerable sheets of this rock, laid bare in the Mitchell quarry near Quiiicy, 

 splitting along w^hat I believe to be the plane of stratification, markings indis- 

 tinguishable from ripple marks If these syenites were of igneous origin, 



if they had been poured out before the deposition of the adjacent stratilied 

 Ijeds, or thrust through them in a state of fusiun, we should expect to find the 

 usual marks of such actions. In the first of tliese cases the later sedimentary 

 deposits would be found lying uucuaformably upon the syenite without any 

 indication of transition ; in the second we should expect to hud a clear line of 

 contact between the syenite and the sedimentary rocks, such as is always to be 

 found where an intrusive mass of trappean nuitter cuts mure ancient rocks. 

 What WT. do find is that the imperfect bedding of the deeper portions of the 

 syenite 1)ecomes more and more clearly defined as we pass towards the exterior 

 of the mass, and gradually passes into unquestiona])ly scdioKuttary rock. 

 Every stnge of tins transition is not clearly seen, but enough is visible to satisfy 

 any one tliat it really exists. Th.e first rocks of (piite unciuestionable stratified 

 origin, lie directly to the north of the Quincy syenite hills, and ctmsist of 



elearly bedded sandstones, approaching (piartzites in their character 



Their general dip is northerly, with a variable angle of inclination which may 



be ruughly averaged at twenty degrees Running the same north course 



across the break, we come upon the lowest of the Braintree series Its 



di]) corresponds with the general inclination observable in the supjiosed strati- 

 fication of the syenite, as Avell as that of the <piartzites inunudiately al)ove it. 

 The whole of this Brahitree scries is fossil iferous, .... and although much 

 changed by metamnrphic action, it is easily pereeived that the whole set of 

 beds contains no trace of shore deposits. Inunediately beyond the exposure of 

 the Braintree beds at Ilayward's Lauding, a dislocation has brought the thin 

 bedded (partzites again to the surface. The alteration in these is so 'great that 



