22 GUIDE TO LOCALITIES. 



Commencing at the southern end of Nantasket beach we find in 

 Long Beach rock, which projects into the sea from the base of 

 Atlantic hill, a bed of conglomerate overlain by (1) a thin layer 

 of finely laminated greenish tuff of jaspery hardness ; and (2) a 

 very complete and typical flow of melaphyr some sixty feet in 

 thickness. This flow is dense and crystalline in the lower and cen- 

 tral portions, while the upper part is highly scoriaceous and shows 

 fluidal lines ; the wiiole recording a quiet submarine or littoral erup- 

 tion which was preceded by explosive action projecting into the 

 water a limited amount of impalpably fine dust. Ascending At- 

 lantic hill from the beach we find that this eruption was followed 

 by a series of eruptions which were alternately quiet and explosive, 

 forming flows of melaphyr and beds of tuff and agglomerate ; the 

 whole being capped by a flow (possibly composite) three or four 

 hundred feet in thickness, which became very generally brecciated 

 by continuing to flow after it began to harden. 



The west side of the small beach between Atlantic and Central 

 hills presents a good section of this volcanic series. One of the 

 flows exposed in this section has preserved very perfectly its orig- 

 inal wavy or undulating surface ; while another, which expands 

 rapidly eastward, encloses many bomb-like masses of amygdaloidal 

 melaphyr. The great bed of melaphyr of Atlantic and Central 

 hills is continued southward in Willow Ledge hill, where it en- 

 closes a very typical bed of tuff ; and eastward in Gun Rock, sev- 

 eral half-tide ledges, and the northernmost of the Black Rock islets. 

 The principal Black Rock islet and several neighboring ledges con- 

 sist of porphyrite of decidedly felsitic character, and probably 

 mark approximately one of the ancient volcanic vents of the 

 Nantasket region. 



The diabase dikes of this shore present some points of special 

 interest. They belong chiefly to two approximately east-west sys- 

 tems, those trending north of east being the older, as proved by a 

 very clear intersection on the shore east of Gun Rock ; and dikes 

 of both these systems are cut by a north-south dike in the conglom- 

 erate at the northern base of Green hill. The largest of the three 

 dikes on Long Beach Rock outcrops again on the shore near Gun 

 Eock as a composite dike, consisting of six parallel branches. Sev- 

 eral dikes are well exposed on Gun Rock. One of these cuts with- 

 out faulting a remarkably regular quartz veinlet ; and encloses in 

 porphyritic fashion minute fragments and single crystals derived 



