Vol. 57.] JIESOZOIC ROCKS IX THE ISLAND OF AliRAN. 227 



even of the important faults affecting them — a conclusion which is 

 fully borne out by the alteration produced upon all these environing 

 rocks at their junction with the eruptive masses. 



The igneous rocks are made up partly of fragmental volcanic 

 materials and partly of various intrusive masses. The area in 

 which the fragmental rocks occur, measuring 2| by 2| miles, is 

 confined within an almost unbroken ring of intrusive igneous rocks 

 chiefly of acid character, and is invaded by numerous sheets, bosses, 

 and dykes of varied composition, some of which are in visible 

 connection with the rocks of the margin. Two smaller areas of 

 fragmental material also occur, one surrounded by the marginal 

 igneous rocks, the other enclosed between them and the outside 

 sedimentary strata. 



The pyroclastic material varies greatly in texture and composition. 

 In places it is comparatively fine-grained, and made up of a paste of 

 felspathic debris, in which occur lapilli, bombs, and large masses of 

 various igneous rocks ranging from basic to highly acid composition, 

 and both of volcanic and plutonic character, the acid varieties being 

 by far the most abundant. In addition to these igneous fragments, 

 fine debris and masses of sedimentary rocks like those of the en- 

 vironment appear in the agglomerate. In places, these increase to 

 such an extent that the felspathic paste due to igneous debris 

 disappears, and rocks are produced which might at first be mistaken 

 for portions of the surrounding formations. Especially is this the 

 case where the conglomerates of the Lower Old Red Sandstone have 

 contributed large numbers of pebbles of quartzite and andesite, 

 though a closer inspection shows that a great number of the derived 

 pebbles are broken. Large masses like the sedimentary rocks, and 

 evidently derived from them, are also found embedded in the agglo- 

 merate, and partly or wholly enveloped in the intrusive igneous 

 rocks. 



But the most remarkable of these inclusions consist of masses of 

 shale, marl, limestone, and sandstone which belong to formations 

 not now found in situ in the island. One of these, several acres in 

 extent, occurs near the edge of the area ^ mile north-east of 

 Dereneneach, and is well exposed in a section cut by the AUt nan 

 Dris, a tributary of the Machrie Water. It consists of dark shales 

 and limestones, associated with and apparently underlain by grey 

 marls, which in turn appear to be underlain by red marls re- 

 sembling those forming the highest visible zones of the Trias in 

 the island. From the dark shales and thin limestones of this mass 

 Mr. Macconochie, in May 1900, obtained a suite of Rhaetic fossils, in 

 which he found a form referred by him to Avicula contorta. The 

 mass, which is thus proved to be of Triassic age, is bounded on the 

 east and north by agglomerate, and on the west and south by felsite 

 and granophyre, which there form the margin of the volcanic area. 



Another large fossiliferous mass of shale associated with impure 

 limestone, partly embedded in agglomerate and partly in intrusive 

 igneous rock, occurs near the head of Ballymichael Glen, | mile 

 > due south from the top of Ard Bheinn. In this mass Mr. Peach 



