218 Scientific Proceedings, Royal Dublin Society. 



this rock was observed, although most of it exhibits well-marked tnie bedding 

 with comparatively thin layers, occasionally hei'e and there well-marked false 

 bedding occurs, showine; that at times at least there was running water when 

 the formation was being deposited. I was fortunate enough to find in one 

 place in this locality the clear line of junction between this rock and the 

 tertiary rock, and the rock we ai-e now considering was found to lie uncon- 

 formably on the latter ; in fact, it would seem that it was deposited in a huge 

 basin scooped out of the older tertiary rock. 



It is, as a rule, impossible to see what this fornration rests on, as there is 

 such a heap of rubbish generally at the bases of the cliffs. I noticed, however, 

 all along the locality where it is found that there is a line of springs cropping 

 out from the valley banks about ten to twenty feet above the river. This 

 would indicate that this rock lies on shingle. In one or two places 1 actually 

 found the shingle on which it lay, and it was of the usual pampa type. I have 

 given the name of Buitreras Beds to this formation. I traced it for upwards 

 of twelve miles along the south side of the Gallegos river valley, and it 

 probably runs much further. At the mouth of the Gallegos Chico it is 

 found lying directly under a huge sheet of lava which caps a table three 

 hundred feet high. It is here over two hundred feet thick, and at the back 

 of Bella Vista, a few miles further along, I found it up to four hundred 

 feet above the river valley. The fact that this formation was deposited 

 in a great hollow scooped out of the tertiary rock, and lies on a layer of 

 shingle of the pampa type, indicates that it is of a comparatively recent 

 origin, and was formed after the shingle was spread over the pampas. As 

 the base of this bed is only a few feet above the floor of the present 

 river valley, the pampa must have been cut down at least three hundred 

 feet before the Euitreras bed began to be deposited. At least one out- 

 pouring of the lava occurred during the time when this bed was being 

 formed, and one river terrace came into existence. From these facts 

 it would seem that the Buitreras beds were deposited during inter-glacial 

 times ; and, although some of the deposit may have been formed on the 

 bottom of a lagoon, some at least was formed under running water, as typical 

 current-bedding can be seen in places. It is evident also from the angular 

 fragments of lava found here and there embedded in it that semi-glacial 

 conditions must have prevailed at least during a portion of the time, as these 

 blocks were probably carried by ice and dropped on the bottom of either a 

 lagoon or a large slowly running river. 



The lava sheet, which extends through it as an even, horizontal, compact layer 

 for some miles, proves that there was one temporary interruption in the deposi- 

 tion, if not more, and that during that interruption the bottom of the lagoon or 



