263 



the crystalline rock ; and other important considerations , con- 

 spire to invalidate this view. And it seems most reasonable to 

 suppose that the slate merely dips against an irregular crystal- 

 line wall ; its beds having been broken off unevenly by the 

 pyroxenic rock, during the elevation of the latter, in either a 

 solid or plastic condition; 



One structural feature of the slate, however, is sufficiently 

 plain, viz., the faults by which most of the transverse dykes 

 are accompanied. The downthrow appears to be sometimes to 

 the south-west, but it is usually to the north-east. Several of 

 these fractures are well marked ; and, on account of the incli- 

 nation of the strata, the outcrops of particular beds, as, for 

 instance, the calcareous bands, experience considerable lateral 

 displacement ; the throw in some instances amounting to several 

 hundred feet. In consequence of the transverse faulting, the 

 limestone beds, in their south-westerly extension, lie beyond 

 the present shore line, ending very abruptly on the land ; and 

 the north-west border of the slate is step-like, the beds on the 

 north-east lying farthest to the north-west. 



Marhlehead Neck. — It is not generally known that this 

 rocky peninsula, which may be regarded as lying on the 

 extreme outskirt of the Boston basin, includes beds probably 

 referable to the same horizon as the slate and conglomerate on 

 the south and west. Briefly stated, the facts are as follows : 

 near the middle of the north-west shore of the Neck, visible 

 only at low tide, is a hard, whitish, fine-grained sandstone or 

 arenaceous slate ; it is evidently largely feldspathic, and turns 

 yellowish on weathering. Porphyritically interspersed through 

 the rock are clear, almost transparent, rhomboidal crystals, 

 one-eighth to one-fourth of an inch long ; these have been 

 examined by Miss Hattie A. Walker, and proved to be ortho- 

 clase. In loose masses of the rock which were thoroughly 

 weathered I have found these crystals changed to a green, 

 soft, unctuous, and opaque mineral, probably pinite. The 

 sandstone has a very gentle dip to the north-west, or toward 

 the harbor ; and it overlies unconformably the banded petro- 



