374 L. V. Pirsson — Geology of Gonanicut Island. 



in addition from weathering that it would be difficult if not 

 impossible to distinguish former granite from the conglome- 

 ratic grit occurring there if it were not for the large feldspar 

 phenocrysts with crystal log rap hie outlines which serve to 

 differentiate it (additional to the included shales in the grit). 



Aplite. — This presents the usual panidiomorphic mixture 

 of orthoclase and quartz with a very little plagioclase, a little 

 iron ore and the ferro-magnesian minerals almost wholly want- 

 ing. It h?s suffered greatly from pressure. Remains of micro- 

 pegmatite structures can be detected at times. 



Micro-pegmatite which occurs as mentioned in the central 

 apophysis is very typical and comparatively fresh. It does 

 not appear to have suffered so much from dynamic forces as 

 the foregoing rocks. 



Mineite. — As before mentioned there are two dikes of this 

 lamprophyre rock on the lower portion of the island as shown 

 on map No. 1. The best and most unaltered exposure is found 

 at Hull's Cove. The rock here is of a dark reddish rusty 

 color on exposed surfaces, stained by iron oxide. On a fresh 

 break it is of a dark gray, and fine grained, in which pheno- 

 crysts of biotite, at times several mm. in diameter, may be seen. 

 Small fragments of the shales through which it has broken up 

 may at times be found in it altered to a siliceous hornstone. 

 Owing to the dynamic pressure the rock has received it breaks 

 with a somewhat schistose fracture at times. The exposure of 

 the second and southern dike at Lion's Head is similar only as 

 the dike is smaller it has suffered more severely from squeez- 

 ing and has a more pronounced schistose structure and the 

 biotite phenocrysts are more rarely seen. The other exposure 

 of this dike at the south point of Austin's Hollow shows these 

 points in a still higher degree, the rock is completely schistose 

 and is recognized only as a dike by its cutting across the strike 

 of the shales, by its same reddish weathering color and by its 

 strike being such that it must be the continuation of the Lion's 

 Head outcrop. These points of identification are confirmed 

 by the microscopical examination. The outcrop of the 

 northern dike on Austin's Hollow was not visited by the 

 writer but is given on the authority of Dr. Foerste. Under 

 the microscope the freshest and best example of these rocks, 

 from Hull's Cove is seen to be composed chiefly of orthoclase 

 and biotite, the latter in two generations. Apatite, zircon, 

 pyrite in well formed idiomorphic crystals and pretty abundant 

 in amount ; plagioclase and calcite are also largely present. 

 Some secondary quartz occurs. The structure where it has 

 not been destroyed is panidomorphic or allotriomorphic 

 granular. 



