372 REPORT — 1888. 



element of a common mineral, such as plagioclase, amongst abundant 

 elements of another character, may be considered to be a fragment. This 

 will not, of course, apply to such minerals as are always more or less 

 sporadic, and are known to be products of alteration, such as garnets. 

 The most important distinction is the following. The outline of a frag- 

 ment may be rounded or angular, and the shape in the latter case maybe 

 peculiar, especially if the rock has been compressed ; but when the ele- 

 ments are forming authigenetically in a rock the crystallisation will sefc 

 up at two or more neighbouring spots, and the growth of the crystals 

 will continue till they meet. The line of junction in this case may be 

 and often is an exceedingly irregular one, curving in and out like the 

 lines of junction of two bones which ossify from distinct centres. In 

 the case of bones such junctions are called sutures, and we may therefore 

 conYcniently describe the similar junctions between crystals as siitural, 

 and define this well-known peculiarity by stating that sutural junctions 

 are characteristic of authigenetic crystals. (See fig. 2.) 



The distinction between fragments of sedimentaiy origin and those 

 produced by crushing is in general easy if the crushing has taken place 

 subsequently to the crystallisation, for it can very seldom happen that 

 absolute resistance and absolute yielding to the crush can be found side 

 by side ; hence crushed fragments are in groups obviously once united 

 into a larger element, but fragments of sedimentary origin are entirely 

 independent ; and the same difierence may also be of assistance when 

 crystallisation has succeeded the crush. 



Lamination may be confounded with foliation when the latter is 

 parallel to the bedding, as seen on the large scale ; but the essence of 

 lamination is the alternate occurrence of difierent kinds of mineral sub- 

 stance in the bands. Now, unless there has been confusion of the sub- 

 stance of a rock, far beyond the point of contorting the laminfe, these 

 original differences will have their effect upon the result even when the 

 rock is crystalline, for one kind of crystal will be more abundant in 

 alternate bands, and the least easily crystallisable will remain as dusty- 

 fragments, which will also lie scattered between the crystals along the 

 same lines. Such phenomena, which are quite distinct from foliation 

 or the orientation of crystals, are taken to indicate original lamination. 

 (See fig. 3.) The principal case in which doubt arises is when the rock, 

 though composed of a uniform substance, is more separable along the 

 line of deposit, and clean crystallised quartz has segregated out in this 

 direction and given rise to an apparent alternation. 



The RocJis Composed of Uniformly Small Elements are probably much 

 more numerous in the oldest series in Anglesey than would be gathered 

 from a comparison of the numbers described in the present report, since 

 few of them would tempt microscopical examination. Yet, excluding those 

 which contain larger fragments, 34 out of 168 of the sedimentary rocks 

 examined have this character. In the Western district this type of rock 

 is largely developed. In four instances it consists of practically unaltered 

 dust. One of these is an indurated purple shale or slate, well bedded, and 

 passing into chloritic schists. It occurs on the east side of the Holyhead 

 Straits (7). There is a certain amount of irregular parallelism in the 

 dust, and there are a few irregular cracks filled with quartz ; but otherwise 

 there is nothing to show that the rock has suffered any change beyond an 

 ordinary compacting by superincumbent pressure. This is a remarkable 

 case as occurring in the lower part of the seriefa The other three belong 



