712 J. W. JUDD ON THE SECONDARY ROCKS OP SCOTLAND. 



contain them. Precisely the same is true of the Middle-Lias 

 strata of Ardnamurchan, which have suffered in an equal degree 

 with those of the southern part of Skye from their proximity to the 

 central axis of a great volcano. 



In Mull, however, the Middle-Lias beds are exposed in an un- 

 altered condition both at Tobermory, on the north side of the island, 

 and at Carsaig, on its south side. At both localities the beds of 

 the Pabba Series exhibit their usual characters, while the overlying 

 Scalpa strata appear in a somewhat different condition from that which 

 they assume in the more northern area. In Mull the Scalpa beds 

 are less calcareous, and consist of greenish and yellow sandstones 

 containing far fewer fossils than their equivalent beds to the north- 

 ward. In their upper part they graduate upwards into white sand- 

 stones and grits, often containing much carbonaceous matter. These 

 changes are suggestive of the prevalence of more littoral, sometimes 

 perhaps even passing into estuarine, conditions in the southern part 

 of the area in question. 



The most northern localities at which the strata of Middle -Lias 

 age have been detected are on the eastern shores of Trotternish, in 

 Skye, and the great cliff-sections of Eaasay. At both these localities 

 their presence was pointed out by Sir Roderick Murchison, who, in 

 drawing his sections, has fallen into some confusion through mis- 

 taking parts of the Middle Lias for Inferior Oolite, the two forma- 

 tions being represented in this area by beds of remarkably similar 

 mineral character. But along the shores at Prince Charles's 

 Cave, and again on the south side of Portree Harbour, as well as in 

 the great mural precipices of Tor Inivaig, we find numerous expo- 

 sures of both the members of the Middle-Lias series. Owing to the 

 numerous slipped masses and the inaccessibility of these precipitous 

 cliffs, however, it is very difficult to obtain satisfactory continuous 

 sections ; and the difficulty is increased by the fact that the strati- 

 fied rocks are often broken up and interrupted by intrusive sheets of 

 basalt, and by the circumstance that their strata are subject to rapid 

 and frequent variations in mineral characters within very short 

 horizontal distances. This will be well seen by comparing the 

 several fragmentary sections adduced by the late Dr. Bryce and 

 Professor Tate (see Quart. Journ. Geol. Soc. vol. xxix. p. 317), which 

 nevertheless serve to give a good general idea of the nature and mode 

 of succession of the beds of this part of the Hebridean Lias. Prof. 

 Tate has done good service to Scottish geology by recording the 

 numerous species of fossils occurring in these strata on the east 

 coast of Skye. 



On the east side of the island of Eaasay the Middle-Lias strata 

 are magnificently developed, the total thickness of the deposits of this 

 age being there probably not less than 500 feet. By the action of 

 the great fault the beds are repeated so that the higher strata (the 

 Scalpa Series) are found extending along the shore for a considerable 

 distance on its south or downthrow side. The fossils are here very 

 numerous alike in the sandy limestones, the calcareous shales (which 

 often contain great concretionary nodules of limestone), and the 



