290 HENRY S. WASHINGTON 



as well as iron oxides, and the large loss is difficult to account 

 for, assuming that "the analysis is correct. There cannot be 

 enough alkalies to make up the deficiency, and it is probably 

 largely water. The rock is possibly decomposed, since Merrill 1 

 has shown that diabase loses silica through decomposition. It 

 is also possible that it is a monchiquite. 



Labradorite '-porphyry. — Closely related to the diabases are a 

 few dikes distinguished by the presence of prominent pheno- 

 crysts of plagioclase in a black, fine-grained groundmass. The 

 best example is Shaler's Dike 175, which cuts across the quarry 

 pit at Pigeon Cove. It is eighteen feet in width, with a strike of 

 N. 9 W. 2 The phenocrysts here are very large and automor- 

 phic. A similar dike cuts the tinguaite at Pickard's Point, in 

 which the phenocrysts at the center are even larger, attaining 

 diameters of more than six inches ; toward the borders they are 

 smaller, and at the contact very small. 



The groundmass of these rocks is like that of the diabases, 

 though an ophitic structure is less often developed. It is com- 

 posed of labradorite, augite, and magnetite, primarily, but in 

 every case is more or less altered, so that secondary hornblende 

 and biotite with chlorite, etc., are present in abundance, and any 

 analysis would be unsatisfactory. 



EXTRUSIVE ROCKS 



Rhyolite. — The only flow rocks found in Essex county are 

 rhyolites, which occur in large sheets about Lynn, Newbury, Old 

 Town, and Marblehead Neck. The last is the only locality which 

 I have visited. This is not the place to dwell upon the discussions 

 which have taken place as to the origin of these rocks, between 

 Sterry Hunt and his followers, who tried to show that these, as 

 well as all the igneous rocks of the region, were altered sedi- 

 ments, and the other party, headed by Wadsworth and Diller, 

 who finally overthrew this view and proved conclusively that 

 they are typical volcanic flows. For particulars of this discus- 



1 G. P. Merrill, Bull. Am. Geol. Soc, Vol. VII, p. 349, 1896. 



z Shaler, op. cit., pp. 592, 607. 



