808 G. W. Tyrrell — Petrology of Arran. 



Petrography. — On the shore of Whiting Bay and at Kingseross 

 Point are numerous rounded pebbles and boulders of a fresh doleritic 

 rock which is rendered pi'ominent by a distinct spotting of rounded 

 lighter areas on a darker ground. Ultimately this rock was traced 

 to a thick dyke penetrating the Triassic sandstones on the shore of 

 Whiting Bay, a little to the north of the School. 



In thin section the rock is seen to consist of an ophitic plexus of 

 plagioclai^e and augite, in which the former is largely predominant. 

 Fresh olivine is very abundant, and a skeletal or platy ilmenite only 

 less so. Analcite or a fibrous radiating zeolite fills many of the 

 polygonal interspaces between the felspar laths. The plagioclase is 

 euhedral and zonal, the composition ranging from acid labradorite 

 (Ab;^ An;^) in the interior to oligoclase (Ab^An^) in a thin marginal 

 zone. As usual, the undulose extinction due to zoning is very 

 prominent in untwinned sections cut parallel to the clinopinacoid. 

 The augite is a deeply-coloured purplish variety, with a pleochroism 

 ranging from deep purplish-brown to a pale sepia, and is probably rich 

 in titanium oxide and alkalies. It is very subordinate in quantity 

 to the plagioclase, and is cut up by the latter into thin strips and 

 small triangular or polygonal patches. The abundant olivine is very 

 fresh, or at most has suffered only incipient serpentinization, 

 and occurs in small, more or less rounded, subhedral granules, 

 which are frequently enclosed along with ilmenite in the plates of 

 augite. Turbid but still isotropic analcite fills up many of the 

 straight- sided interspaces between the felspars. In some cases, 

 however, the spaces are filled with a low-polarizing, radial zeolite, 

 whose optical characters agree with those of scolecite. The ilmenite 

 is mostly skeletal, but sometimes occurs in long thin plates or rods, 

 which are enclosed in and earlier than the titanaugite, but frequently 

 mould the terminations of felspar laths. Occasionally small scraps 

 of red biotite are associated with the iron-ore. The spotting visible 

 in the hand specimen is due to a comparative scarcity of the 

 ferromagnesian minerals in certain areas, and is only recognizable 

 with difiiculty in thin section. From this description it will be 

 evident that this rock is identical with crinanite. I have been able 

 to confirm the identification by comparison with slides of typical 

 crinanite from Argyllshire kindly^ lent by the Geological Survey. 



The great sill of Dippin which has been described by Corstorphine 

 as olivine-analcime-diabase,^ and by Barker as teschenite,- appears to 

 me to be more nearly related to crinanite. It agrees with the latter 

 in the perfect ophitic relations between augite and plagioclase, and 

 in the abundance of fresh olivine, both of which characters ai'e very 

 unusual in tlie teschenites. In many specimens (e.g. from the north 

 end of the Dippin cliff) the interstitial analcite is not at all prominent. 

 Only in some outcrops in Glen Ashdale does the analcite become 

 as abundant as in the teschenites. The mass of the Dippin rock, 

 however, is much coarser in grain than is usual with the typical 

 crinanites, but this, of course, is conditioned by its mode of occurrence 



^ Tschervi. Min. Petr. Mitth., xiv, 463-5, 1895. 



■ The Geologrj of North Arran, South Bute, and the Cumbraes (Mem. 

 Geol. Surv.), 1903, pp. 112-14. 



