298 EDSON S. BASTIN 



a mile long and one-half mile wide, showing coarse pegmatitic textures 

 throughout. Most commonly the pegmatite masses are roughly 

 lens-shaped and lie parallel to the foliation of the inclosing schistose 

 rocks, their attitude being dikelike or sill-like, according as the 

 schists lie at steep or gentle inclinations. 



The rocks associated with the pegmatites are granite gneiss, 

 granites of various textures, and schists of sedimentary origin. The 

 field relations show that the pegmatites are invariably intrusive into 

 the sedimentary schists, frequently cutting sharply across the schist 

 foliae though usually intruded parallel to them. Characteristic 

 contact metamorphic minerals are sometimes developed. Into the 

 granite gneisses the pegmatites are also in some instances distinctly 

 intrusive, but in other cases their relations indicate that the two rocks 

 are nearly contemporaneous and probably related in origin. 



The relations between the pegmatites and the granites indicate 

 beyond reasonable question that the two rock types are genetically 

 related. Evidence of this is found (i) in the fact that the predomi- 

 nant minerals in both rocks are the same; (2) in the occurrence of 

 granite in all districts where pegmatites are found, and (3) in numer- 

 ous observed instances of transition from granite to pegmatite. One 

 of the most instructive instances illustrating such transition is exposed 

 on the shore of Boothbay Harbor, and is illustrated in Fig. i. Micro- 

 scopic examination of the fine-grained granite and the pegmatite in 

 this occurrence shows that the mineral species in the two rocks are 

 identical, the sole differences being in the texture and the proportions 

 of the constituents. In other instances small irregular segregation- 

 like masses with pegmatitic texture are wholly inclosed by normal 

 granite. Although in certain instances distinct dikes of pegmatite 

 cut the granites and in other instances dikes of granite cut the peg- 

 matites, there is no evidence that the two rocks are of widely different 

 ages or that there was more than one general period of granite and 

 pegmatite intrusion. The granites are known to be of late Silurian 

 or early Devonian age, and it is probable that the pegmatites are 

 to be similarly correlated. With the exception of certain diabases, 

 the granites and pegmatites are the youngest known igneous rocks 

 of the state. 



Mineral composition and texture. — As already stated, the dominant 



