J. W. JTTDD ON THE SECONDABY ROCKS OE SCOTLAND. 719 



e. The Loiver Oolite. 



The series of strata representing this formation in the Western 

 Highlands is of very considerable thickness and importance. Rocks 

 of this age must, indeed, have originally had a very wide distribu- 

 tion ; for traces of them are found at various points, from the Shiant 

 Isles in the north to Ardnamurchan in the south. The best expo- 

 sures of their strata, however, are those which occur in the Islands 

 of Skye and Raasay, in the sections so well described by Sir Roderick 

 Murchison, and afterwards in greater detail by the late Dr. Bryce 

 and Mr. Tate. 



The Inferior Oolite, where best developed in the "Western Isles, 

 has a thickness of probably not less than 400 feet. An exact esti- 

 mate of the dimensions of the whole formation and of the proportions 

 of its several members is, however, rendered difficult owing to the 

 manner in which the numerous intrusive sheets of Tertiary dolerite 

 are interspersed through the series, and interrupt the continuity even 

 of its best sections. The formation may be denned as consisting of 

 the following members, proceeding from above downwards :• — 



feet. 



I. Limestones, almost wholly made up of comminuted shells, and re- 



sembling in their mineral characters the English Cornbrash and 

 Forest Marble, with which formations they were identified by Sir 

 Roderick Murchison 45 



II. Beds of white sandstone, with some subordinate shaly bands, the 



whole containing much carbonaceous matter and some plant- 

 remains, includin g both Ferns and Cycacls 60 



III. Alternating beds of sandstone and shale, the sandstone being 

 usually calciferous, and containing great spherical concretions, 

 and sometimes passing into shelly limestones. Marine fossils occur 

 in these beds, but are by no means common, except in the highest 

 of them, which abounds in Belemnites of several species. Ferns, 

 Cycads, wood, and plant-remains of various kinds occur through- 

 out the whole series 160 



IV. Sandy micaceous shales, alternating with calciferous sandstones, 

 the latter containing numerous spherical concretions and exhi- 

 biting mamillated surfaces. There are also some bands of shelly 

 limestone in the series. Fossils, all of which are of marine forms, 

 abound in these beds 120 



"With respect to the correlation of these different members of the 

 Lower Oolite, it may be remarked that the division I., although made 

 up of fragments of shells, seldom contains specimens in a sufficiently 

 perfect state of preservation to permit of their specific determination. 

 No Cephalopods have been found in this part of the series ; and the 

 few species of Lamellibranchiata and Brachiopoda which it has 

 yielded, though all of Lower Oolite age, are not sufficient to enable 

 us to state absolutely whether the bed should be regarded as belong- 

 ing to the Great or to the Inferior Oolite series. 



The division II. is probably of estuarine origin, and there is no 

 direct palseontological evidence concerning its exact position in the 

 geological series. It is clear, however, that it is on the same horizon 

 with one or other of the Lower-Oolite Estuarine series of the English 

 Midland Counties or of Yorkshire. 



Q. J. G. S. No. 135. 3 b 



