150 



eral, which it seems necessary to regard wholly as a product of 

 the alteration which the rock has undergone. During the 

 elongation of the pebbles, or subsequently, while they were yet 

 subject to the action of the mechanical and chemical forces by 

 which their forms were distorted and constitutions altered — 

 for, as Prof. Hitchcock has distinctly pointed out, there is little 

 room to doubt that the conditions were favorable for chemical 

 change, and that there must have been considerable interchange 

 of material, especially alkalies, among the pebbles, and be- 

 tween the pebbles and paste, — the material of the more feld- 

 spathic pebbles appears to have passed from the compact or 

 granular to the crystalline state. It is certainly conceivable 

 that forces competent to develop a schistose structure in a con- 

 glomerate largely composed of pebbles of quartzite, granite, 

 and epidote, might induce conditions favorable for the partial 

 crystallization of the semi-plastic materials. 



Briefly stated, the transformation, where complete, appears 

 to be as follows : the pebbles, as such, disappear, leaving a 

 schistose or foliated structure ; the greater part of the paste, 

 and probably a portion of the pebbles, are converted into a 

 distinct mica or hydro-mica ; and, finally, crystalline feldspar 

 and vitreous quartz appear in the substance of some of the 

 original pebbles. Thus the pudding-stone is changed to some- 

 thing which is not easily distinguishable from a coarsely 

 schistose, 1 micaceous gneiss, such as abounds in the neighbor- 

 ing towns of Blackstone, Mendon, and Uxbridge. And it is 

 highly interesting to observe that these same gneisses possess 

 some of the most striking structural peculiarities of the altered 

 conglomerate. For instance, I have repeatedly observed in 

 the gneiss that a transverse vertical section shows no schis- 

 tosity, no stratification, only a coarse, granitoid aspect ; while 

 on a strike section the schistose structure stands out promi- 



1 1 employ the term schistose here, as elsewhere throughout this work, as a name for 

 the texture which a rock possesses when composed of limited, lenticular, and over- 

 lapping layers; although I am aware that most geologists give the word a different and 

 wider meaning, allowing it to stand as a general name for the texture of any dis- 

 tinctly stratified, crystalline rock. 



