EASTERN MASSACHUSETTS. 409 



varieties of syenite were six, of which only the first five occur in the dis- 

 trict under discussion here. It would seem that he iuchided under the 

 name syenite many diabases as well as hornblendic granites. The divis- 

 ion of the rock into parallel portions observed in one locality on Cape 

 Ann he regards as pseudo-stratification (concretionary structure), and 

 not real stratification. The syenite was said to pass by insensible gra- 

 dations into granite on one side, and into greenstone or porphyry on the 

 other, the author remarking as follows : — 



" Or when these rocks are wanting, some of the stratified rocks, such as 



hornblende slate, graywacke, or new red saudstone repose upon it In 



all cases where this rock occurs, we find it between the oldest granite and 

 greenstone, or the earlier stratified rocks. Hence I infer that a portion of the 

 materials of which granite is composed, imder certain circumstances were con- 

 verted into sienite, and that these circumstances existed generally in that por- 

 tion of the melted granite nearest the newer stratified rocks. Or if we suppose 

 it erupted at a different epoch from the granite, cert^iin causes always forced it 

 upwards between the granite and the newer rocks. Or if we suppose it to 

 have resulted from the meltiug down of the stratified rocks, then perhaps their 

 more or less perfect fusion produced the difference which we find between 

 granite and sienite." 



The syenite is said to penetrate " sienite of a different variety, or 

 greenstone." President Hitchcock thought that the evidence in favor 

 of its igneous origin was not strong, yet he held that view. (L c, 

 pp. 451— IGo.) A second and revised edition of this report was published 

 in 1835, but his views remained essentially unchanged. 



In 1838 was published President Hitchcock's " Report on a Re- 

 examination of the Economical Geology of Massachusetts." In this the 

 existence of the Lynnfield serpentine was pointed out (pp. 137, 138). 



President Hitchcock's Final Report was published in 1841, and in it 

 certain changes were made. The graywacke was subdivided into the 

 Coal Measures, Old Red Sandstone, and Graywacke. The last com- 

 prised the conglomerates about Boston Harbor and in some other local- 

 ities (l. c, p. 534).- The argillite in the vicinity of Boston was still 

 included in it, although lower in the series. Under the head of " Meta- 

 morphic Slates" were classed among other formations "Aggregates of 

 Porphyry," " Varioloid Wacke," and the " Flinty Slate of Nahaut." Of 

 the first he says : — 



" The best example . . . ., perhaps, is in Hingham, a little west of the vil- 

 lage ; and in Cohasset, at the head of Xantasket Beach. At the latter place is 

 a coarse breccia, or conglomerate, which is chiefly made up of fragments of por- 

 phyry reunited by a cement of the same materials, and is sometimes almost 



