484 Geology of Galway and Mayo. 



Attention is directed to the foliation of the metamorphic sedi- 

 mentary rocks, which seems to have been predisposed to follow the 

 most marked structure in the original rocks. Simple or parallel 

 lamination, apparently, was the principal structure of the ancient 

 rocks of the district, so generally the foliation is parallel to the 

 bedding. Curled or spheroidal foliation likewise is present, which 

 in some cases seems to have been caused by the original rock con- 

 taining nodules. Other forms of foliation are pointed out. 



The Silurian rocks, supposed to be the equivalents of the 

 Llandovery beds, consist of green and grey grits, sandstones, and 

 shales, many of which are fossiliferous ; with, near the base of the 

 formation, red and purple shales, sandstones, and conglomerates. 



The Carboniferous rocks are also described, and their fossils 

 noticed, in the account of the districts where they occur. 



The Drift-deposits seem to belong to three distinct classes ; the 

 first being the true Boulder-clay, a slightly sandy clay, in which are 

 embedded beautifully polished, scratched, and rounded blocks and 

 fragments of rock ; the second, Boulder, or Moraine-Drift, consisting 

 of broken and sub-angular fragments and blocks of local rocks 

 (rarely polished), mixed with sandy clay; and the third, Esker- 

 Drift, or "Post-Drift" gravels and sands. The first and third occur 

 on the low country west of the lakes, and the last seems to be the 

 Boulder-clay washed or sifted, as it is found merging into a Eocky- 

 drift, or half- washed Boulder-clay, and from that into the Boulder- 

 clay itself. The second occurs principally in the valleys, or on the 

 slopes of the mountainous district. A similar drift in Wales, is 

 supposed by Professor Eamsay to have been formed by the glaciers 

 that once existed in its mountain valleys. That such glaciers did 

 also exist here appears probable, as will be seen in the table of ice- 

 strise given in this " explanation." 



The relations between the form of the ground and its internal 

 structure are alluded to. At least three great periods of disturbance 

 and denudation are considered to have affected the country from 

 early geologic times down to the Glacial epoch. The most ancient 

 of these was that which affected the metamorphic and granitic rocks, 

 as on the east of Benlevy the Silurian rocks were deposited in places 

 against a cliff over 200 feet in height. The second was that during 

 which a great part of the Silurian rocks and also parts of the older 

 rocks were carried away. The third period of denudation was that 

 in which so much of the Carboniferous rocks was excavated and 

 carried away, as to leave the features of the country to a great extent 

 as it is now found. This may have been partly due to marine action, 

 but the last great agent at work would seem to have been ice, as all 

 the rock-surfaces which are protected from the " meteoric abrasion," 

 now in progress, are well polished, planed, and striated ; moreover 

 on the exposed surface of the country, the authors have noted the 

 rounded flowing outlines which are now universally recognized as 

 the result of ice-agency. Since the ice has disappeared the existing 

 denuding agencies have been, and are daily, at work, slowly and 

 gradually changing the features of the country. Adjoining the lakes, 



