J. W. JUDD ON THE SECONDARY ROCKS OF SCOTLAND. 



725 



the slipped masses of the overlying basalts. The upper divisions I. 

 and II. resemble the equivalent strata of Skye and Haasay, but 

 appear to have a somewhat greater thickness ; while the lowest 

 member, III., not represented in the northern localities, has in the 

 southern area great thickness and importance. 



The upper argillaceous series, which is fairly well exposed in 

 Eigg, but much better in Muck, contains, as in Skye and Eaasay, 

 thick beds of dwarfed oysters (0. hebridica, Forbes), indicating a gra- 

 dual transition into the marine Oxfordian beds above. Both in the 

 northern and southern areas the argillaceous and arenaceous divi- 

 sions of the Great Estuarine Series exhibit — as is so constantly the 

 case with formations of this character — rapid and sudden changes in 

 composition and thickness within very short distances. 



The most southern exposure of the strata of this formation 

 which is certainly known occurs in the island of Muck ; and, as it 

 has not been before described, I add a few notes on the section. 



Eig. 9. — Section exposed in reefs seen at low water at Camus Mhor, 

 in the Island of Muck. 



a. Grass-covered slopes. b. Shingle. 



c. Great bed composed of shells of Ostrea hebridica, Forbes. 



d. Laminated shales, with bands of hydraulic limestone. 



e. Oyster bands, with alternating limestones and shales. 



/. Shales and limestone containing Cyclas, fish^remains, &c. 

 g. Calciferous sandstone (the lowest bed exposed). 



The strata in question occur at the head of the little bay of 

 Camus Mhor on the south side of the island, and consist of a 

 number of beds forming reefs along the shore, with a dip inland of 

 5° (see fig. 9). These strata, of which probably the thickness exposed 

 is not more than 20 or 30 feet, consist of thick masses of oysters 

 intercalated with shale and limestone bands about 8 feet thick, lying 

 upon beds of shale and limestone containing Cyclas, Cyrena, and 

 some univalves. The lowest beds seen are somewhat more sandy in 

 character. It is clear that all the beds of Camus Mhor in Muck be- 

 long to the division I. of the Great Estuarine Series, and it is not 

 improbable that under the green slopes and shingle of the shore 

 above traces of the overlying Oxford-Clay strata might be found. 



It is quite possible that among the greatly altered rocks lying in 

 close proximity to the gabbro-masses of the volcanic axis of Ardna- 

 murchan the Great Estuarine Series may be represented ; but that 

 such is actually the case I have not been able to obtain any clear 

 and unmistakable evidence. 



These estuarine strata, so conspicuously exposed in Sutherland 



