368 EEPORT— 1888. 



trict is divided into two — the Eastern and the "Western portions — by a line 

 running along the west boundary of the grey gneiss. 



EOCKS OF SEDIMENTARY ORIGIN. 



The study of these has reference, first, to their mineral constitution ; 

 secondly, to their original structure ; and, thirdly, to their alteration. 



1. The Mmerals of the Sedimentary Bocks. — The greater number of 

 these may be said to belong to the acid type, the ordinary constituents of 

 basic rocks being comparatively rare. 



Quartz is the most abundant mineral of all. If we except the lime- 

 stones and other special rocks, there is hardly another in the whole series 

 in which, when the minerals are recognisable, qnartz is not among them. 

 It occurs both authigenetically and as derived fragments. In the latter 

 case it sometimes contains needles of apatite, as in the quartzite of Holy- 

 head Mountain (1), and sometimes fine needles of rutile, as in the slates 

 of Roscolyn {66). It usually has numerous bubbles, which mostly occur 

 along cracks, and these cracks are often continuous from one element 

 to another, and must therefore have been produced in the quartz after 

 it had been placed where it now is. Some of the derived fragments of 

 quartz, however, have their lines of cracks terminating at their boundaries. 

 Occasionally the silica along broad bands is chalcedonic, as at Peniel (11), 

 and more rarely small rounded fragments of bluish tint may indicate opal. 



Felspar. — In the absence of any structural lines it is often impossible 

 to do more than determine the family. When felspar and quartz are 

 associated, and both are of minute size, it is often difficult to distinguish 

 between them. In the quartz the inclosures may generally be recognised 

 as bubbles, in the felspar as dust or small crystals. When, however, both 

 are water-clear and too small to give any reaction in convergent polarised 

 light they can only be distinguished by the slightly higher refractive 

 index of the quartz, which makes it stand out from the felspar when both, 

 are together. The larger crystals of felspar present a very charac- 

 teristic phenomenon. (See fig. I.) They are filled with minute flaky or 

 tabular small crystals. These may be arranged in intersecting lines, or 

 be quite irregular. They are often of considerable relative size, as in the 

 derivative fragments in the mica schists of the Eastern district (151). 

 Whether the small crystals are original inclusions or results of decom- 

 position is difficult to say, but their general character and arrangement 

 seem to indicate the latter. Such 'speckled' felspars are not always in- 

 determinable, but are often banded plagioclase ; they are almost invariably 

 derivative, or at least of an older generation than the bulk of the rock. 

 This speckling seems to be a further stage to ' schillerisation.' 



Plagioclase. — The banding which is presumed to characterise the 

 plagioclastic group of felspars may have more than one origin. Pro- 

 fessor Judd ^ has recently suggested that in some cases it is brought 

 out by pressure. There are numerous instances in these Anglesey rocks, 

 as at Bodlew (148), of the bauds being continuous across only half of the 

 crystal. In tbese cases they are less regular and distinct. When two 

 such series of lines cross at right angles we get the phenomena of micro- 

 cline ; but in this also there is great irregularity— the bands die out and 

 change position as the slide is rotated. Such are seen in the grey gneiss 

 of Gwalchmai (69) and at Ty Gwyn (188), but they are better shown in 



' Mineraloffical Mag. 1888. 



