354 G. W. Tyrrell — Tertiary Dykes 



discovered all over the Clyde area. The true crinanites are 

 inseparably associated with, and may be shown to pass into by 

 a graduated series of sections, ophitic olivine-dolerites, which differ 

 from them only in the absence of analcite or radial zeolites. The 

 group is typically non-porphyritic, free from glass, and rich in 

 olivine. In general the rocks are so fresh that it is difficult to 

 believe that the interstitial analcite and zeolites can be other than 

 primary constituents. The crinanites are ophitic olivine-anaicite- 

 dolerites of mafelsic composition, i.e. the felspars are quantitatively 

 approximately equal to the combined olivine, augite, and iron-ores. 

 The chemical analysis of a typical crinanite from Inver, Jura (Flett, 

 1911), gives a norm which falls into the subrang auvergnose of the 

 American Quantitative Classification, but very close to ornose (III, 5 

 (3)4. '5). It shows 1*5 per cent of nepheline, and consequently 

 the rock falls into the "unsaturated" group of Shand, whereas the 

 tholeiites are quartzose and fall into the " over-saturated" group. 



A typical example from Whiting Bay is described in the above- 

 cited paper. The felspar is much in excess of the augite, so that the 

 small plates of the latter mineral are cut up into thin, detached, 

 triangular strips by the felspar laths. The augite is always the 

 deeply coloured, brown-violet, titaniferous variety. The olivine is 

 generally quite fresh, and occurs in rounded grains which are 

 uniformly distributed over the field. The interstices are filled with 

 analcite or radial zeolites, but the amount is always small, much 

 smaller than in the teschenite group. When the olivine happens to 

 be serpentinized, the green material appears to migrate into the 

 interstices and obscures the presence of analcite. Some of the dykes 

 may thus be set down as olivine-dolerites when they are really 

 crinanites. The rocks vary in coarseness of texture from almost 

 gabbroid to basaltic types, according to the size of the intrusion. 

 They usually occur as broad, massive dykes, which may in some 

 cases have feeder relations to the great crinanite sills of the south of 

 Arran. Besides the Whiting Bay dyke mentioned above, crinanites 

 are now known to occur in Arran at the following places : 

 Kingscross Burn, a quarter of a mile south-west of Kingscross 

 Bridge ; by the high road above Cordon, Lamlash (Fig. 3b) ; on the 

 shore south-east of Cordon, Lamlash ; the shore a quarter of a mile 

 north of the Pier, Brodick ; Invercloy, Brodick ; and An Sgriob 

 Quarry, 2 miles south of Corrie. Related olivine-dolerites occur at 

 the first fall in Glen Ashdale, Whiting Bay, and on the shore near 

 Dunfion, Corriegills. 



In Bute a !N".-S. crinanite dyke extraordinarily rich in fresh olivine 

 occurs on the road 2 miles north-east of Dunstrone ; and a 3 ft. N.N.E. 

 dyke on the shore at Kerrytonlia is probably also a crinanite, but 

 contains much diffused serpentinous matter. The related olivine- 

 dolerites also appear in several places. In the great Cumbrae two 

 crinanite dykes are known, and two of the related olivine-dolerites 

 (Tyrrell, 1917). Both here and in Bute the dykes of this group 

 trend in a N. to S. or N.N.E. direction. On the Ayrshire mainland 

 crinanites are known from the shore near the Heads of Ayr. 



Conclusion". — Although much still remains to be discovered 



