22 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 22, 



thickness of fully 150 feet, the series was suddenly brought to a 

 close by a protracted accumulation of sandy mud, abounding in 

 Gryphcece, which now forms the shale-reefs of Obe Breakish. This 

 muddy sediment had ceased to darken the water, and the abundance 

 of animal life had spread a limy floor below the sea, when, like a pre- 

 monitory symptom of the changes of after-times, the long ridge of 

 Beinn Shuardail was upheaved. Its summit, exposed to the dash of 

 the waves, was ere long bared of the capping of lias-beds under which 

 it rose, and fragments of limestone, red sandstone, and quartz-rock, 

 some well-rounded, others sharp and angular, rolled down the sides 

 to form the limestone-breccia already described. The reef, wasted 

 by the surf, began at length to sink together with the surrounding 

 ocean-bottom. Limestones formed over its site, many of them richly 

 charged with the characteristic organisms of the Lias, and often 

 seamed with thin courses of shale. And when they had attained a 

 depth of not less than 200 feet, a change in the direction of the cur- 

 rents, or some other modifying cause, brought the calcareous series 

 to an end. Dark sandy mud settled down on the floor of the sea, 

 entombing in rich profusion the remains of Ammonites, Belemnites, 

 Pectines, Giyphcea:, Pinnce, Pentacrinites, and many others. The 

 thickness attained by these argillaceous deposits fully doubled that 

 of all the preceding beds, and my excursions have not yet carried me 

 further than the shales of Scalpa, which are not probably the close 

 of the series. These latter strata seem to be the equivalents of the 

 Marlstone of England. It remains as the labour of future years to 

 carry on this history through the Upper Lias and Lower and Middle 

 Oolites that fringe the eastern shores of Skye, as far as its most 

 northern promontory — the pillared cliffs of Duntulm. 



The next point in the geology of Strath to which the evidence 

 conducts us, is the eruption of the igneous rocks. I have said that 

 there is nothing in the district itself to indicate how long after the 

 deposition of the Scalpa-shales the first of these eruptions took place. 

 That is a question which a careful examination of the northern part 

 of the island may do much to solve. Meanwhile we are in posses- 

 sion of some important facts in the history of the igneous eruptions 

 of the Hebrides. The district of Strath affords convincing evidence 

 that at a period posterior to the Middle Lias, there was an extensive 

 outburst of basalt, which, in the form of dykes, penetrated the liassic 

 strata throughout their entire area, without, however, effecting any 

 marked alteration of them ; that at a subsequent time a tract of 

 syenite fully forty square miles in extent, along with several minor 

 masses, broke through the beds, tilting them on end, and producing 

 in them a widely diffused and complete metamorphism ; and that 

 at a still later period a series of syenitic and greenstone eruptions 

 forced their way among the limestones and shales, not, however, tilt- 

 ing them, nor giving rise to any extraordinary amount of alteration. 



Future labours in other parts of Skye, and among other islands 

 of the Hebrides, will probably throw much light on the history 

 and age of these volcanic rocks. The Western Islands, unquestion- 

 ably, must have been the seat of powerful igneous action during the 



