Tol. 68.] PBE-CAMBKIAN ANB CAMBEIAN OF PEMBROKESHIEE. 



389 



The general characters of the rock lead us to the conclusion that 

 it should take its place among the augite-diorites rather than 

 with the epidiorites, for we believe the hornblende to be in the 

 main primary. 



The felspars are nearly all albite, but they are frequently decom- 

 posed, chiefly to micaceous aggregates. The presence of a little 

 epidote, however, may point to the presence of a more basic soda- 

 lime-felspar in the undecomposed rock. 



A striking feature of this diorite is the abundance of sphene 



Pig. 2. DyTce-rocks from the pre-Cambrian and Cambrian. 



A. B. 



E7232 



E8991 



A. Albitized and silicified olivine-dolerite cutting the upper sandstones of the 



Welsh Hook Beds of Cwm Mawr, Newgale. The figure shows a well- 

 shaped pseudomorph after olivine in a fine-grained ground-mass (magnified 

 25 diameters). 



B. Albitized and chloritized ophitic dolerite cutting the tuffs of the Pont-yr- 



hafod Grroup. The figure shows laths of albite in a mass of chlorite and 

 iron-ores (magnified 25 diameters). 



which occurs in large ophitic plates moulded on both felspars and 

 hornblende. It is almost colourless, and appears to be an original 

 constituent of the rock. The presence of abundant sphene in soda- 

 rich plutonic rocks is not an unusual feature, and seems to point to 

 some intimate relation between the titanic acid and the soda-content 

 of the original magma. Ilmenite is present in some quantity, and 

 on decomposition has given rise to granular sphene. Apatite occurs 

 in fairly large prisms, but is an uncommon accessory. 



