NEIGHBOTJKHOOD OF SAKN, CAERNARVONSHIRE. 



449 



long ridge of Mynydd-y-Rhiw to a height of 1000 feet, is quarried 

 further north near Cadern-y-groes, and extends in a north-north- 

 easterly direction to Tyn-y-coed, half a mile south of Sarn village. 

 Besides this, several sheets and dykes occur just west of Sarn ; and 

 further west, at Ty-rutten, a tongue-shaped mass runs with the strike 

 of the strata, leaving a narrow strip of shales between itself and the 

 granite (fig. 3). This mass must abut at its southern extremity on 



Fig. 3. — Section from Pen- Craig to south of Sarn. 

 (Length about | mile.) 



S. Shales, Upper Arenig or Lower Bala. 



G. Granite. 



D. Diabase, tongue at Ty-ratten, dyke and sheets near Sarn. 



/. Probable faults. 



the tongue of granite a little north of Muriau ; but unfortunately 

 their junction, on the right slope of the little valley, is not exposed. 

 There are numerous diabase sheets and small bosses in western 

 Lleyn, which will not be considered here, though some of them, 

 such as the sheets at Llanfaelrhys and Tyn-y-rhedyn, have probably 

 a cognate origin with the diabase of the Sarn district. 



The rocks are sufficiently alike to be described together, and 

 present no very striking peculiarities. In hand-specimens the rock 

 appears as a diabase of medium grain, greenish from decomposition- 

 products, and showing distinct crystals of felspar. 



The microscope shows the original constituent minerals to be the 

 usual iron-ores, plagioclase felspar, and augite ; the secondary pro- 

 ducts, leucoxene, calcite, zeolites, quartz, chloritoid minerals, and 

 ferric oxide, do not require special notice. The magnetite and 

 ilmenite, constantly the first- formed minerals, are fairly plentiful in 

 rods and skeleton crystals. One or other of these constituents is 

 always present, and frequently both in the same slide : there are 

 not wanting signs of an intergrowth of the two minerals, or per- 

 haps of magnetite and titaniferous magnetite. 



The plagioclase occurs in lath-shaped or, at least, elongated sec- 

 tions, showing twin-lamellation on the albite-law. In the dykes 

 and sheets near Sarn the felspars are often once twinned only, but 

 in general a fine striation is seen between crossed nicols. The ex- 

 tinction-angles agree with those of oligociase and labradorite. 



The augite is very pale brown or almost colourless in thin sections, 

 with the prismatic cleavage pronounced, and rarely traces of a pina- 



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