PRE-CAMBRIAN ROCKS OF NEWFOUNDLAND 473 



Clarenville on Trinity Bay a mass of gabbro is intruded by granite, 

 and both in turn are cut by younger dikes of aplite, rliyolite, and 

 porphyrite dikes. 



The hornblende granite gneiss shows a cataclastic texture and 

 is believed to be the oldest rock in the district. Its outcrop was 

 noted in only three areas of very small extent along the border of 

 the Holyrood granite batholith. 



The gabbros occur as small, irregular patches on the borders of 

 and within the area of the Holyrood granite batholith, and also as 

 huge dikes in the Conception slate series. They are predominantly 

 of a blackish or dark speckled gray color weathering to an ashen 

 gray. In grain they vary from fine to medium, and pegmatitic 

 facies are entirely lacking. Mineralogically there seem to be two 

 types, which can be distinguished by microscopic methods alone: 

 one characterized by hornblende without augite; the other an 

 augite gabbro in which the pyroxene may be unaltered or partially 

 altered to hornblende or uralite. 



Microscopically the augite gabbros are found to be pre- 

 dominantly hypautomorphic-granular in texture, occasionally 

 becoming ophitic. The augite and plagioclase may be fresh, or 

 partially or completely altered ; and the alteration of these minerals 

 seems to have proceeded independently, for fresh augite may be 

 associated with altered plagioclase, and vice versa. The plagio- 

 clase may be partially altered to sericite, or flecked with sericite 

 and partially replaced by quartz or epidote. The augite may be 

 partially or completely altered to compact green hornblende or to 

 an aggregate of uralite fibers or of uralite, quartz, and chlorite. 

 The ilmenite is usually altered to leucoxene and is very small in 

 amount. Pyrite is rare and magnetite relatively abundant. The 

 peak of Holyrood Butterpot is formed of an elHptical mass of 

 gabbro about 1,200 feet by 500 feet, completely surrounded by 

 granite. It is peculiar in having quartz in small amount filling the 

 interstices between the feldspars in such a manner as to indicate 

 its origin as an original constituent, the last product of crystalliza- 

 tion. Some of the augites are twinned, and others show idio- 

 morphic basal sections. A chemical analysis of this rock is given 

 on page 476, No. 2. 



