308 REPORT OF NATIONAL MUSEUM, 1904. 



In the consideration of the application of Beaumont's theory of 

 mountains to those of Massachusetts, Hitchcock thought to tind six- 

 systems of strata and contemporaneous uplift in the State. These six 

 systems as enumerated are: (1) The oldest meridional system, (2) the 

 trap system, (3) the latest meridional system, (4) the northeast and 

 .southwest system, (5) the east and west system, and (6) the northwest 

 and southeast system. 



The Hoosac Mountains, considered under his oldest meridional sys- 

 tem, he regarded as due to two different epochs of elevation, the last 

 taking place after the deposition of the new red sandstone, as was 

 .shown by the slight dip (15° to 20°) of the latter, while the slates and 

 gneisses of the range stand nearly vertical. The force which gave 

 rise to mountain uplift he regarded as gravity acting on the crust as 

 the nucleus gradually shrunk away in process of cooling. 



Not the least interesting chapters in Hitchcock's work are those 

 relating to the unstratified rocks. Here, under the name of greenstone, 

 he included the entire series of basic and intermediate eruptives now 

 classed as melaphyrs, diabases, and diorites. Their mineral composi- 

 tion was stated, however, as hornblende and feldspar, though the min- 

 eral identified as hornblende has been proven by modern microscopic 

 methods to have been almost wholly augite. Columnar, compact, 

 amygdaloidal, and porphyritic varieties were recognized. 



Concerning the igneous origin of these rocks he professed little 

 doubt, and he accounted for their appearance in the form of continuous 

 sheets on the assumption that they were poured out, not from craters 

 but from linear openings produced by the shrinkage of the earth's 

 interior. Thus early was recognized the phenomena of fissure erup- 

 tions, as later developed by Geikie and others. 



Commenting on the occurrence of the greenstone in "veins,' 1 as at 

 Nahant, he expressed the opinion that the slate in which these veins 

 occurred could not have been solid at the time of the intrusion, this 

 on account of the size of the fissures, which seemed to him to be too 

 wide to have been formed by desiccation. Evidently the possibility 

 of the formation of these fissures by dynamic agencies acting on con- 

 solidated materials was not then realized. 



Under the name of porphyry he included the compact eruptive rocks 

 now classed mainly as aporhyolites or quartz porphyries, such as form 

 the cliffs at Marblehead, and are also found in other localities, as Hing- 

 ham and Maiden. Their porphyritic structure he thought could 

 result only from igneous solution, The base of these porphyries he 

 stated to be of compact feldspar, which he regarded as formed by the 

 melting down of preexisting feldspathic rocks. 



The slaty structure sometimes shown, and which is now recognized 

 as flow structure, he regarded as representing the original structure 

 of the slaty rock from which the porphyry was derived by fusion. 



