410 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



reconverted into compact porphyry At the head of Naiitasket Beach 



is another metainorphic rock, Tying contiguous to the breccia juHt mentioned. 

 . . . . I incline to the opinion that it was originally a hard slate, like that on 

 Nahant, and the Brewster Isluudy, which haw been very much changed and 

 lillcfl up with veins oC epidote, by tlie action of heat. Some of it appears as if 

 converted into a sort of compact feldspar." (/. c, pp. 547, 548.) 



The latter rock he described in the Beport for 1833 (p. 255) as con- 

 sisting of ''fragments of gray and yellowish green compact feldspar, 

 united by an unknown dark-colored cement." 



The " varioloid wacke " is the amygdaloid of most writers on the 

 geology of Eastern Massachusetts, and, as in 1833, President Hitchcock 

 holds that it passes into and " is only a metamorphic variety of the 

 graywacke formation." (l^inal Ecport, 1841, pp. 548, 54D.) In this 

 Ecport he held that 'Hhe difi'crent unstratiticd rocks appear to be the 

 result of volcanic agency exerted at different periods under different cir- 

 cumstances," and gives reasons for this view, which in part at least 

 appear t(j be sound. lie further remarks : — 



F 



*' The greater degree of crystallization in the older unstratified rocks may be 

 explained, by supposing a more perfect fusion of tlie materials tlian in recent 

 lavas, and greater slowness in cooling, under perhaps the more powerful pres- 

 sure of a deep ocean." (/. c, pp. 71)0, 791.) 



He advances two theories for the origin of the '' Primary Stratified 

 Bocks." 



1. "The stratified primary rocks are merely tlie detrital or fossiliferous 

 rocks altered by heat. As these accumulated at th.e bi)ttoni of the ocean, being 

 much ])oorer conductors of heat than water, they would confine the internal 

 heat that was attempting to escape by radiation, until it became so great as to 

 bring the matter into a crystalinc state : but not great enough to produce en- 

 tire fusion, so as to destroy the marks of stratification." 



2. "This hypothesis supposes the primary stratified rocks to have been 

 formed partly in a mechanical an.d partly in a chemical mode, by aijueous and 

 i'qieous agency, when the temperature of the crust of the globe was very high, 



and before organic beings could live upon it." 



President Hitchcock seems indeed to incline towards the second 



theory, (l c, pp. 79G-798.) 



In 1839 iMr. AVilliam Prescott's '' Sketch of the Geology and Mineral- 

 ogy of the Southern Part of Essex County, in Massachusetts," was pub- 

 lished. (Jovu^nal of the Essex Co. Nat. Hist. Soc, 1852, L, pp. 78-91.) 

 The rocks were described as gneiss, syenite, greenstone, porphyry, silicioua 

 breccia and brccciated porphyry, puddingstone, amygdaloid trap, and 



