192 PROCEEDINGS OF THE GEOLOGICAX SOCIETF. [Jan. 8, 



have operated upon these strata since their deposition, and the con- 

 stant filtration of water through them, at once suggest themselves 

 as the causes of this phenomenon. The calcareous masses of the 

 matrix, which are sometimes sufficiently pure to he burnt for lime, 

 are made up of drifted, waterworn, and comminuted shells, corals, 

 spines of Cidaris, &c, with many fragments of wood, and greatly 

 resemble the rock of the Forest-marble. The larger fossils, which 

 remain unbroken, Belemnites, oysters, and corals, usually show signs 

 of having been drifted and waterworn. Among the great extra- 

 neous blocks we find numerous trunks of trees completely fossilized ; 

 some of these are many feet in length ; and with them occur many 

 beautiful stems of Cycads. 



§ 4. The Included Blocks of the "Brecciated beds." 



The careful study of these masses of stone enclosed in the midst 

 of the Upper Oolite strata of Sutherland and Caithness furnishes us 

 with the following details : — 



(a) Form. — Some of the blocks of stone are perfectly angular, 

 and on being cleared from the investing matrix exhibit the charac- 

 ters of the surfaces of fracture as clearly and distinctly as if their 

 separation from the parent rock took place but yesterday. These 

 perfectly angular masses constitute the majority of the blocks ; but 

 there also exist in great numbers subangidar fragments of rock, the 

 edges of which exhibit signs of attrition, and which have been evi- 

 dently subjected, for a limited period, to degrading forces in a stream 

 or on a shore. Lastly, not a few of the fragments, especially among 

 those of smaller size, are completely worn and polished into pebbles. 



(&) Size. — The variation of the included blocks in this respect is 

 very remarkable. Sometimes the aspect of a fractured surface of 

 one of the " brecciated beds " is that of an angular gravel, numerous 

 small fragments of foreign rocks being cemented together by sand 

 or shelly detritus. More usually the masses are of much larger 

 size ; and, as has been already pointed out, the appearance pre- 

 sented by the beds is that of rough walls, such as are often seen in 

 mountainous districts, composed of angular blocks of the most various 

 size. Occasionally, however, masses are found included in these 

 remarkable "brecciated beds" of such prodigious dimensions as 

 altogether to startle the observer, and bewilder him not a little in 

 seeking for an explanation of the phenomenon. Eemarkable ex- 

 amples of this kind are found on the shore opposite to Port Gower, 

 and again between Allt-a-ghruan and Allt-an-aird, south of the 

 Green-Table Point. At the latter locality there is a mass composed 

 of hard light-coloured sandstones, alternating with indurated shales 

 and calcareous flagstones. This mass stands on end, its strata being 

 vertical ; and it forms a singular object among the denuded edges of 

 the highly inclined " brecciated beds " in which it lies, its strike 

 being at right angles to theirs, and its dip wholly discordant. The 

 exposed upper edge of this mass measures 20 feet by 10 feet. A 

 little to the south of this is another similar mass, composed of the 

 same materials, the beds of which are also vertical. The continuity 



