696 J. W. JUDD ON THE SECONDARY ROCKS OE SCOTLAND. 



gists. The results, moreover, of the labours of Murchison, Geikie, 

 and Bryce, as interpreted by the palseontological studies of Sowerby, 

 Forbes, Wright, and Tate, leave us little room for doubt as to the 

 true correlation of the various strata which are seen in the grand sec- 

 tions of Skye and Raasay with the equivalent deposits in England. 

 As these results, accompanied by detailed lists of fossils, are pub- 

 lished at various times in the Transactions and Journal of this 

 Society, and have further been very well summarized in the ' Cata- 

 logue of the Western Scottish Fossils,' the introduction to which is 

 from the pen of Dr. John Young, it will be quite unnecessary for mo 

 to recapitulate the conclusions which have been already arrived at. 

 But, inasmuch as a great number of the less accessible sections of 

 the Jurassic rocks in the Western Islands have hitherto been very 

 imperfectly described, while some have hitherto remained alto- 

 gether unnoticed, it seems to me desirable to consider these in con- 

 nexion with those concerning which we already have a considerable 

 amount of information. By doing so I hope to be able to show 

 what was the distribution of the several members of the Jurassic 

 series in this district, to illustrate the remarkable changes in cha- 

 racter which some of them are found to undergo when traced over 

 considerable areas, and thus to pave the way for arriving at a 

 satisfactory basis on which to found a discussion concerning the phy- 

 sical geography of the district and of the conditions which prevailed 

 in it during the periods when these strata were deposited. At the 

 same time I shall endeavour to correct and supplement the previ- 

 ously published account of the old and familiar sections, whenever 

 such correction and addition may be found necessary. 



a. The Infralias. 



The strata which underlie the Lima- or BucJdandi-beds in Western 

 Scotland acquire at some points such thickness and importance that 

 it will be convenient to follow the example of French authors, and 

 to separate them from the remainder of the Lower Lias under a 

 distinctive title. The formation in question certainly in places ex- 

 ceeds 150 feet in thickness, and includes strata both of marine and 

 estuarine origin. Even the marine beds, however, very commonly 

 exhibit clear evidence of the prevalence of littoral, and sometimes of 

 brackish-water conditions, during their accumulation ; and the whole 

 formation thus constitutes a transition series between the probably 

 lacustrine Poikilitic below and the undoubtedly marine Lower Lias 

 above. The dwarfed forms of the oysters and other shells of the 

 Infralias of this district, and the absence of the Cephalopoda from it, 

 may be cited as warrant for these observations ; but it must at the 

 same time be remembered that some purely marine beds, crowded 

 with corals and univalves, also occur in its midst. 



The most conspicuous and characteristic strata of the Infralias 

 are certain very hard bluish-grey limestones, sometimes compact, 

 at others subcrystalline, and occasionally oolitic in structure. 

 These limestones are not unfrequently found to be crowded with 



