1875.] JTJDD THE SECONDARY ROCKS OF SCOTLAND. 157 



land was either marked by the deposition of estuarine strata, or is 

 totally unrepresented, is rendered highly probable by the fact that, 

 among the numerous Jurassic fossils derived from the drift, I have 

 never been able to find a single example of the very characteristic 

 and well-marked Upper Lias forms. 



§ 6. The Loiver Oolite. 



In Sutherland the place of the Lower Oolites is occupied by 

 a thick mass of sandstones, shales, and coals, exhibiting many evi- 

 dences of deposition under estuarine conditions. This series of 

 strata is directly and conformably overlain by a great thickness of 

 marine strata, representing the lower portions of the Middle Oolite. 

 The great series of estuarine beds is thus proved to be older than 

 the Middle Oolite by the fact that the lowest of the marine strata 

 is, as we shall show hereafter, a bed representing the Kelloway 

 Rock, and containing a fauna which enables us to refer it without 

 doubt to the lowest zone of the Middle Oolite. That the estuarine 

 beds, in part at least, represent the Lower Oolites, is confirmed by 

 the fossils found in one of the marine beds included in the series. 

 We have already pointed out that there are reasons for supposing 

 that this estuarine series, which is certainly of great thickness, and 

 of which the base has never been reached, may represent the Upper 

 Lias as well as the Lower Oolite. There is, unfortunately, no point 

 at which the relations of this set of beds with the known Liassic 

 strata of the district can be observed. 



These estuarine strata, as might be anticipated from their mode 

 of origin, are very inconstant in character, so that sections at short 

 distances from one another often exhibit surprising differences in 

 the order and thickness of the strata passed through. The highest 

 beds of the series were very carefully studied by Mr. Alexander 

 Robertson, who laid accounts of his observations before this Society 

 in 1843 * and 1846 f. 



The following is a generalized section of this series of beds so far 

 as it is known at Brora : — 



ft. in. 

 " Roof-bed" of the coal. Base of the Middle Oolite (marine). 



(1) Main bed of coal. Sometimes a well-formed coal of good quality, 



at others approaching more in character to a lignite. In places 

 it is seen to be wholly made up of the crushed stems of Equi- 

 setites columnaris, Brongn. This bed of coal contains in its midst 

 a baud of pyrites about 6 inches thick, which greatly detracts 

 from the value of the seam. Maximum thickness 3 6 



(2) Beds of black highly carbonaceous shale, often crowded with plant- 



remains, and alternating with thin bands of crushed shells ( Unto, 

 Cyrena, Perna, Ostrea, &c, and Cypriotes). In some of the beds 

 the scales and teeth of fishes abound (Lepidotus, Semiotus, Pho- 

 lidophorus, Hybodus, Acrodus, &c.) ; and these occasionally form 

 thin bone-beds. This series of beds includes in its upper part 

 several beds of coal, varying in thickness from 16 inches down- 

 wards ; but these do not appear to be so constant as the main 



* Proc. Geol. Soc. vol. iv. p. 173. 



t Quart. Journ. Geol. Soc. vol. iii. (1847), p. 113. 



