208 Br.M.E. Wadsicorih — Syenite and Gahhro in Massachiiseifs. 



region, and hence, for convenience of description, the rock of the 

 syenite djkes may be styled micro-syenite. 



The relation of the syenite proper to the common hornblendic 

 and lepidomelane granite ^ of the region (the syenite of Hitchcock, 

 and other American authors) has not yet been made out distinctly. 

 Both are eruptive, and heretofore have been taken to form the same 

 rock-mass. In fact, the syenite is partly in the areas mapped as 

 belonging to the Norian or "Naugue Head" formation, and partly iu 

 that mapped as belonging to the Huronian. Near West Manchester 

 the granite rises obliquely in a dome-shaped mass from beneath 

 the syenite, but the actual contact was not seen ; and elsewhere, 

 when the two rocks were in juxtaposition, good contacts were not 

 found. The difficulty in determining their relations was further 

 increased by their close resemblance to each other macroscopically, 

 and the fact that the micro-granite and micro-syenite dykes, by 

 which the granite and syenite are respectively cut, are closely alike. 

 So far as seen, the granite occupies the coast northward from near 

 " Manchester-by-the-Sea," and the interior of the country going 

 southward from that point. At least, an examination of all the 

 railway cuttings between Manchester and Salem revealed syenite at 

 only two of them, and those were near the sea-shore. The syenite, 

 however, occupies a large portion of the coast-line between Manchester 

 and Salem. The preponderance of evidence is that the granite is the 

 younger rock, unless it is contemporaneous with the syenite. 



Gahhro. — On Woodbury's Point, Beverley, occurs a gneissoid 

 schist which is probably the oldest rock in the vicinity. This is cut 

 by irregular dykes of gabbro which generally run approximately 

 parallel to the foliation of the schist. This gabbro varies much in 

 structure in different portions even in the same dyke, three different 

 types being found with various intermediate grades, 1. A compact, 

 tough, milk-white or waxy felspathic mass, containing a few grains of 

 diallage, and closely resembling many specimens of the Volpersdorf 

 gabbro. 2. A crystalline rock composed of approximately equal 

 amounts of felspar aiid diallage, and in structure is like a coarsely 

 crystalline diabase, or the common crystalline gabbros like those 

 from Eaduthal and Neurode, 3, A rock in which the felspar becomes 

 subordinate, serving merely as a matrix to hold the large porphyritic 

 crystals of diallage, some of which are two or three inches long. 

 From the alterations observed by the writer in the gabbros from this 

 place, San Domingo, and other localities, he has been led to believe 

 that many of the coarsely crystalline diorites of these regions have 

 been formed from the alteration of gabbro, in which the diallage has 

 been replaced by secondary hornblende. The before-mentioned 

 syenite was found cutting the gabbro and holding fragments of it, 

 while the micro-syenite contains diallage fragments at the places in 

 which it traverses the gabbro. 



In the granite of Davis Neck, Bay View, Gloucester, a variety of 

 gabbro occurs quite abundantly. This is a grey, coarsely crystalline 

 rock, with predominating felspar. The felspar is in approximately 

 1 Proc. Boston Soc. Nat. Hist. 1878, vol, xix. pp. 309-316. 



