207 



by the weathering-out of soft pockets on flat surfaces there 

 are so-called human ''footprints" to be seen, as often happens 

 in cases of this kind. 



Along the coastal escarpment the consolidated alluvia 

 rest unconformably on the dolomite, or lowest member of the 

 Cambrian limestones. In some places the silicified grits are 

 seen to fill the joints in the Cambrian dolomite on which 

 they rest, forming pseudo-dykes, varying in width from 1 foot 

 to several yards. Near Dinham's they are in close proximity 

 to the Tertiary cherts, which they apparently overlie. The 

 upper limits of the deposits are estimated at about 300 feet 

 above sea level. 



The occurrence of two sets of siliceous grits in close 

 proximity was at first a little confusing, notwithstanding the 

 great disparity which exists in their respective ages, and it 

 required a general examination of the geological area before 

 definite conclusions could be reached with respect to their 

 stratigraphical relationships. It is, therefore, not to be 

 wondered at that Mr. Tepper, in his pioneer papers on the 

 geology of the neighbourhood, should have failed to recognize 

 the distinction which exists between the two sets of grits, 

 and placed them in one category as "Ardrossan grits." If 

 examples taken from the respective beds be promiscuously 

 mixed together it becomes almost impossible to discriminate 

 which examples are Cambrian and which are Pleistocene. 

 The newer beds are often the more siliceous and ancient look- 

 ing. Apart from the evidence in the field the chief points 

 of distinction are : (1) the Cambrian basal grits are usually 

 more or less arkose in their composition, including a propor- 

 tion of clastic felspar, while in the alluvial grits there is 

 an absence of felspar and the granular constituents are repre- 

 sented almost exclusively by quartz; and (2) the Cambrian 

 grits are distinctly bedded, sometimes laminated and fissile, 

 while the alluvial grits present no fissile bedding planes and 

 form spheroidal and solid siliceous masses or massive rocks 

 with irregular weathering. This difference is recognized in 

 a practical way in the neighbourhood, for whilst the basal 

 grits are commonly used for building purposes the con- 

 solidated alluvial grits are never so used. 



2. Consolidated Sandstones near Sea Level. The occur- 

 rence of silicified fossil wood at Rocky Point, 12 miles south 

 of Ardrossan, was brought to my knowledge by Mr. O. W. 

 Thompson, of Ardrossan, whose uncle, now deceased, had 

 made the discovery many years ago. Under Mr. Thompson's 

 guidance I made two visits to the spot. 



Rocky Point (pi. xxvi.) is a notable headland, about 

 80 feet in height. The upper portions of the cliff consist 



