FETTKE, MANHATTA]^ schist OF" new YORK 231' 



of the water, boric, carbonic and hydrosulphuric acids, the fluorides, 

 €hlorides and borates of the alkali metals and of the rare earths along 

 with some of the silicates, free silica and other oxides which remain be- 

 hind after the greater portion of the magma itself has solidified. This 

 "mother liquor" is later extruded through fissures developed in the cooling 

 mass and the pegmatites are, therefore, found as dikes in the igneous rock 

 from which they were derived and in the adjacent wall-rock. 



The exceedingly coarse crystalline texture and accompanying struc- 

 tures of the pegmatites are due to the presence of the gases and mineral- 

 izers in the magma from which they crystallized. Just what per cent of 

 the entire amount of the residual magma they represent is hard to say. 

 Professor Iddings*"^ states that the proportion of gas present probably does 

 not amount to more than ten times that present in the original magma 

 from which the pegmatitic "mother liquor^' was differentiated. From 

 this it varies greatly down to cases where it is the same as the parent 

 magma. A medium-grained granite or aplite then results. 



The pegmatites of southeastern New York State are undoubtedly re- 

 lated genetically to large batholithic masses of intrusive granite. It is 

 highly probable that the large areas of granite previously described which 

 have been so extensively uncovered by erosion in western Connecticut rep- 

 resent these intrusive masses. The area farther to the west is very likely 

 also underlain by other batholiths which have not yet been exposed except 

 in an occasional projecting knob. Where the granite appears at the sur- 

 face in western Connecticut, it often passes, as already mentioned, into 

 coarse pegmatite. The transitions can best be explained by imagining 

 the pegmatites to be injected into overlying but only partially cooled and 

 solidified portions of the original magma. The two would then be very 

 closely related. Also as these granite areas of western Connecticut are 

 approached, the pegmatitic sheets and dikes become very abundant and of 

 extensive size, indicating that there must be some common genetic relation 

 between them. 



The granitic intrusions just described probably accompanied the great 

 orogenic movements which resulted in the intense folding of the rocks of 

 this region, including the Inwood limestone and Manhattan schist. Such 

 periods, as Professor C. E. Van Hise^^ has pointed out, are very favorable 

 for the entrance of igneous rocks. The relations of the intrusive sheets 

 and injected lenses of pegmatite and granite to the mica schist are such 

 that they must in many cases have penetrated the older rocks before the 

 period of folding had come to an end. The sheets and lenses are often as 



** Op. cit., p. 276. 



*5 "Earth Movements." Trans. Wis. Acad. Sci. Arts and F-^tters, Vol. II. 1898. 



