Dr. M. E. Wadsicorth — Syenite and Gahhro in Massachusetts. 209 



square prisms, weathering grey, but often on the fresh fracture having 

 a bluish or pinkish tinge. In many parts this gabbro contains 

 crystals and masses of a pinkish striated felspar (probably labradorite), 

 one piece of which was fifteen inches long by seven inches broad. 

 The gabbro occurs in rounded crater or neck-like masses, and in 

 dykes, one of which is twenty-seven feet in width. The largest 

 gabbro area holds in its centre a pinkish mass composed mainly of 

 the pinkish felspar before mentioned. This mass is nearly round in 

 outline, and from fourteen to fifteen feet in diameter, while it ap- 

 parently widens in depth. This eruptive gabbro is like much of that 

 which has been regarded as typical of the so-called Norian formation, 

 and apparently to a boulder of it found on Marblehead Neck is due 

 the first suggestion of the presence of the Norian formation in 

 Eastern Massachusetts, Boulders both of the Davis Neck and 

 Woodbury Point gabbros have been observed by the writer freely 

 scattered in the drift over Middlesex and Essex Counties, and there- 

 fore these rocks doubtless exist in situ in many other localities than 

 the two mentioned. 



ElcEoUte (^Zircon) Syenite. — The writer has previously pointed out ^ 

 the existence in Marblehead of a zircon syenite like that from 

 EredericksvEem, Norway. The same rock further occurs in Salem, 

 particularly on Salem Neck, where it forms dykes and large masses 

 in the diabase. It was here that sodalite and elaeolite were found in 

 an apparent boulder many years ago, and the writer has discovered 

 that the coarser crystalline portions of the zircon syenite contain 

 elEeolite in quite large and abundant masses, giving rise to the rock 

 known as elaeolite syenite. The zircon syenite is further found on 

 the Beverley shore, 



DijTces. — The entire coast-line is more or less cut by dykes of micro- 

 granite, micro-syenite, quartz-porphyry, diabase, diorite, and mela- 

 phyre. Part of the diabase and melaphyre dykes are non-porphyritic 

 on the sides, but in the intermediate portions they are filled with 

 felspar crystals, varying from minute ones to giant forms six to eight 

 inches in length. Many of these crystals are of irregular outline, and 

 have been penetrated by the ground-mass. 



Trachyte. — A dyke of trachyte of the same kind as that previously 

 described by the wi-iter ^ as forming an overflow on Marblehead Neck, 

 was found on the Beverley shore, near the Manchester boundary-line. 

 This dyke cuts the granite, both walls being found, and at the point 

 observed it was forty-four inches wide. 



Geological Succession. — If the sequence of all the dykes cutting 

 one another be taken, numerous epochs of eruption can be found, but 

 speaking broadly the general order of arrangement would, from the 

 present evidence, appear to be as follows : — 



1. The schistose rocks which seem to form the basis, and to be 

 of sedimentary origin, 2. The older basaltic rocks— gabbro, diabase, 

 and diorite — which have been intruded through the schist and occupy 

 large areas of the country, 3. Syenite. 4. Eleeolite (zircon) syenite, 



' Proc. Boston Sec. Nat. Hist. Feb. 1, 1882. 



2 Proc. Boston Soc. Nat. Hist, 1881, vol. xxi. pp. 288-294. 



DECADE III. — VOL. 11.— NO. V. 14 



