GEOLOGY OF THE LONG LAKE QUADRANGLE 455 



Doubtful gneisses. Here are classed other rocks, differing from 

 the preceding in that they seem to be wholly of igneous origin. 

 They have been equally, if not more, changed from their original 

 condition than have the rocks of the preceding group, and all traces 

 of their original characters haA^e disappeared. Similar rocks, in 

 general not to be distinguished from them, occur associated with 

 the sediments, where they are clearly as young, or younger than 

 they are. So these may represent great masses of such rocks, 

 massed in such amount as to have wholly displaced the sediments. 

 On the other hand they may be, in part, older and represent the 

 rocks of the floor on which the sediments were deposited. The 

 question is, as yet, undecided; the former is the more probable. 



Great igneous intrusions. The rocks of the two preceding series 

 at present found in the district constitute only a fragmentary rem- 

 nant of those formed at this early time. They have suffered large 

 loss from above by surface wear, slow but long continued. They 

 have likely also suffered loss from beneath owing to the attack of 

 masses of igneous rocks which were working their way upward. 

 Prior to the appearance of these intrusions the older rocks seem to 

 have suffered compression and as a restdt to have been much 

 changed in character. At the time of compression they must have 

 been buried under a considerable load of overlying rock, the great 

 masses of the intrusions solidified under large load, and both are 

 now at the surface because of the removal of this overlying rock 

 during long ages of surface erosion. The intrusive rocks invaded 

 the entire district, but Essex and southern Franklin counties felt 

 the full force of the invasion, these igneous rocks forming most of 

 the present surface there, while elsewhere they are not as prominent. 



These igneous rocks may be grouped into four great classes, anor- 

 thosites, syenites, granites and gabbros, all no doubt derived from 

 some great parent molten mass beneath by some process of dif- 

 ferentiation. The anorthosite intrusion was the first and bulkiest, 

 forms the heart of the igneous district, and was followed by smaller 

 and more scattered intrusions of syenite, of granite and of gabbro. 



These rocks have also been profoundly modified by the action of 

 great compressive forces, while deeply buried, but are not so thor- 

 oughly changed in character as the earlier rocks, retaining many 

 traces of their original structures. 



Following this time of igneous intrusion the region seems to have 

 been a land area for long ages and to have undergone a prodigious 



