ON THE GEOLOGY OF ARDROSSAN. 603 



II. Statigraphy. 



The above table gives the general sequence of strata in the north of Ayrshire,* 

 and correlates the various exposures in the neighbourhood of Ardrossan. It should be 

 noted that the cornstones and cement-stones are here very imperfectly developed. The 

 horizon is, however, well indicated either by the occurrence of sandy limestone and 

 greenish marl, or by the presence of a false-bedded pebbly white sandstone with 

 calcareous concretions, which is a well-marked member of the series at Ardrossan and 

 the Horse Island. The volcanic series which at Whitecraigs is about 300 yards in 

 thickness is barely half as thick at Ardrossan, while in the Horse Island the same series 

 is compressed into less than 50 feet.t The Upper Old Red Sandstones are well exposed 

 in the Stanley Burn from Whitelees to within 200 yards of the old quarry at White- 

 craigs. They dip constantly up stream at a low angle, increasing towards the north, 

 and it is probable that both they and the overlying volcanic series originally bent over 

 an anticlinal axis striking N.W.-S.E. behind Ardrossan. The red sandstones formerly 

 quarried in the town may thus be correlated with similar red sandstones at Blackhall. 

 The thin concretionary limestone or cornstone 300 yards north of Burnfoot Bridge, 

 which is here taken as the limit of the Old Red Sandstone, is occasionally exposed at 

 low water after a heavy storm. 



III. Petrography. 



(a) The Carboniferous Lavas and Tujfs. 



Two types of lavas may be distinguished. 



(A) There are on the one hand the lenticular terminations of a few of the great 

 basaltic flows which build the volcanic plateau to the north,J a]Q d which may be seen 

 between tide-marks, upturned at the Inches and dipping out to sea at the Harbourback 

 and the Horse Island. They are usually somewhat coarse in texture and much 

 decomposed. (B) Below these at the Inches and the Castlehill only, not at the 

 Harbourback, there occurs a columnar, fine-grained, and compact dark blue basalt, very 

 vesicular on the margins, which is in every way similar to that which has risen along 

 the margin of the large truncated neck in the Horse Island. It is evident that before 

 the plateau lavas had yet covered the site of Ardrossan there were a number of small 

 volcanic vents to the westward, through one of which, now in part in the Horse Island, 

 a basalt rose which flowed eastwards to form the Castlehill. Since that time the 

 Ardrossan neighbourhood has been much shattered by faulting, but with care it is still 

 possible in this way to correlate the various igneous rocks. 



Thin beds of ash, ashy sandstone, and clay are associated with the lenticular masses 



* See also Mem. Geol. Sur., Expl. of Sheet 22, p. 9. t Of. Ancient Volcanoes of Great Britain, vol. i. p. 393. 



X Ancient Volcanoes of Great Britain, p. 368. 



