PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 199 



Granite.]. 



But perhaps the most interesting feature of this rock is the occurrence of 

 /iiltnnlorite feldspar in secondary growth. It embraces the reddened orthoclastic 

 substance in precisely the same poikilitic manner as does the secondary quartz. It was 

 mistaken for quartz at first, being clear and plainly of secondary date. A section, 

 however, was found cut perpendicular to , on which extinction on a cleavage is 

 33, which, according to the determinations of Fouque, shows labrador-bytownite. 

 Some of these crystals are of considerable microscopic size, but it is but rarely that 

 they are free from the red substance over areas sufficiently large to operate on with 

 convergent light. They are usually simple crystals and when they are cut trans- 

 versely they are seen to run athwart the red fibres, and to spread independently 

 amongst them. In but one instance was seen an albite (or Carlsbad) twinned section. 



The green spicules mentioned as piercing the fibres of the spheruliths is a 

 monoclinic pyroxene. In a section perpendicular to the prism axis, an optic axis is 

 visible, and by means of it the direction of the optic plane is seen to be parallel to a 

 third cleavage, or parting. The latter is hence parallel to the brachypinacoid (010), 

 as in diopside. The long lath-shaped sections, which are more nearly parallel to the 

 vertical crystallographic axis, usually show no cleavages, but are crossed irregularly 

 by transverse coarse cracks which are approximately perpendicular to the prism axis. 



This mineral has high double and single refraction, and in all respects, so far as 

 its characters are ascertainable, it agrees with diopside. 



There is also apparently a little rut He, manifesting its four systems of cleavages, 

 much darkened by iron oxide, and clouded by gray leucoxene. Three sections were 

 made, one at random, through the granular or granophyric portion, one parallel to 

 the prevalent structure, and one perpendicular to it. 



Age. Cabotian. 



Remark. Whatever may be its source, whether from deep-seated acid magma, 

 or from fusion of the elastics of the region, this "red rock" material manifests here 

 distinctly a secondary origin for the quartz, whether micropegmatitic or poikilitic 

 in its manner. There was also at the time of the generation of the quartz, a cotem- 

 porary growth of a basic feldspar which has optic characters, indicating labradorite, 

 which permeated the acid element without uniting chemically with it. The smallness 

 of the masses of this red rock included in the basic eruptive gave occasion for 

 sudden transitions from characteristically basic phenomena to acid, without the lapse 

 of sufficient time, in sudden cooling, for chemical union, and at the same time 

 indicates a limited and probably local origin for the red material. The phenomena 

 here may be compared with the descriptions given by Bayley of the contact 

 phenomena on Pigeon point.* 



* Bulletin cU, U.S. Geol. Survey. 



