OF SOME ROCKS FROM PEMBROKESHIRE. 553 
bably immature, or completely altered felspar ; its habit is that of 
a plagioclase. Augite in minute irregular interstitial grains is 
present and also olivine. This is another diabase. 
No. 39, from Ogof-golchfa, Whitesand Bay. A dull greenish 
and purplish schistose rock brecciated with fine materials, The 
mass of this rock is of exceedingly fine grain, showing between 
crossed nicols a very minutely granular mineral which depolarizes 
light. Here and there are ovoid fragments of microcrystalline 
structure (probably felsitic) and the débris of rocks like Nos. 36 and 
37, but still more altered. An occasional quartz crystal is also 
present. Contains some magnetite and also opacite, which, besides 
being irregularly distributed, lies thick in places along the lines of 
foliation. This is a schistose tuff. 
No. 40 is from the same locality. It is a similar rock to No. 39; 
but is more schistose and contains more of the microcrystalline 
felsites and quartz, with much distributed magnetite. 
No. 41. A pale greenish compact rock resembling a hornstone, 
from Emlych. The constituent of this rock is so exceedingly 
minute and indefinite that its nature cannot be determined even by 
means of the microscope. Seen with a 7-inch objective in ordinary 
light it appears to be schistose, and between crossed nicols is 
distinctly so. It is probably a fine tuff. 
No. 42, from Ogof-llesugn, is a greenish breccia with large 
fragments of quartz, and quartz with felspar. In thin section shows 
much quartz, some clear and some dirty. Contains fragments and 
crystals of two felspars, also distinct fragments of a rock consisting 
of quartz, altered orthoclase, and plagioclase with an interstitial pale- 
green mineral. Some of the fragments exhibit the dendrito-graphic 
structure so frequently found in the rock of the Dimetian ridge. 
The whole appears to be cemented by a kaolin-like paste. There 
can be but very little doubt that we have here a breccia consisting 
almost entirely of the débris of the Dimetian, as the fragments above 
described are not to be differentiated from it. The fragments are ° 
not sufficiently angular to admit of the supposition that they are the 
result of a brecciation 7 situ. , 
Nos. 48-48. These are specimens of the Cambrian conglomerate 
from West Creek, Nun’s Chapel. They consist very largely of 
quartz, but principally of the dirty kind. Some enclose fragments 
of a well-defined quartz-schist, the mica constituent of which is 
pale-coloured and slightly dichroic. There are also fragments of a 
felsite with outline indications of the former presence of crystals of 
felspar, and of a schistose rock like No. 41, quartzite, and traces 
of a mica much obscured by alteration. 
No. 49 is from the Lower Cambrian conglomerate of Porth- 
melyn, near Nun’s Chapel. It consists principally of the dirty 
quartz with traces of felspars, which in some instances have dis- 
appeared during the preparation of the section. This is an arkose 
and probably derived from the Dimetian. 
No. 50. Cambrian grit or conglomerate from near Chanter’s 
