140 Heports and Proceedings — 



The existence of scattered patches of fossiliferous strata, lying be- 

 tween the old gneissic rocks and the masses of Tertiary lava in the 

 Hebrides has been known to geologists for more than a centmy. 

 By Dr. Macculloch, who did so much for the elucidation of the in- 

 teresting district in which they occur, these strata were referred to 

 the Lias ; but Sir Eoderick Murchison showed that several members 

 of the Oolitic series were also represented among them. Later 

 researches have added much to our knowledge of the more accessible 

 of these isolated patches of Jurassic rocks in the Western Highlands. 



During the seven years in which he has been engaged in the study 

 of these interesting deposits, the author of the present memoir has 

 been able to prove that not only is the Jurassic system very com- 

 pletely represented in the Western Highlands, but that associated 

 with it are other deposits representing the Carboniferous, Poikilitic 

 (Permian and Trias) and Cretaceous deposits, the existence of which 

 in this area had not hitherto been suspected ; and by piecing to- 

 gether all the fragments of evidence, he is enabled to show that 

 they belong to a great series of formations, of which the total 

 maximum thickness could have been little, if anything, short of a 

 mile. 



The relations of the scattered patches of Mesozoic strata to the 

 older and newer formations respectively are of the most interesting 

 and often startling character. Sometimes the Secondary rocks are 

 found to have been let down by faults, which have placed them 

 thousands of feet below their original situations in the midst of 

 more ancient masses of much harder character. More usually they 

 are found to be buried under many hundreds, or even thousands of 

 feet of Tertiary lavas, or are seen to have been caught up and in- 

 closed between great intrusive rock-masses belonging to the same 

 period as the superincumbent volcanic rocks. Occasionally the only 

 evidence which can be obtained concerning them is derived from 

 fragments originally torn from the sides of Tertiary volcanic vents, 

 and now fouud buried in the ruined cinder-cones which mark the 

 sites of those vents. In some cases the mineral characters of the 

 strata have been greatly altered, while their fossils have been occa- 

 sionally wholly obliterated by the action of these same igneous forces 

 during Tertiary times. 



In every case the survival to the present day of the patches of 

 Secondary rocks can be shown to be due to a combination of most 

 remarkable accidents : and a study of the distribution of the frag- 

 ments shows that the formations to which they belong originally 

 covered an area having a length of 120 miles from N. to S., and a 

 breadth of 50 miles from E. to W. But it is impossible to doubt 

 the former continuity of these Secondary deposits of the Hebrides 

 with those of Sutherland to the north-east, with those of Antrim to 

 the south, and with those of England to the south-east. From the 

 present positions of the isolated fragments of the Mesozoic rocks, 

 and after a careful study of the causes to which they have owed 

 their escape from total removal by denudation, the author concludes 

 that the greater portion of the British Islands must have once been 



