Professor T. G. Bonney — Eroded Rocks in Corsica. 391 



pointed bill ; it was about a yard long, 16 inches in greatest breadtli, 

 and 14 inches in greatest depth, the lowest point being about 1\ feet 

 from the ground. All the figures are estimates, the hollow being 

 inaccessible. In it were two basins, one in the head (the deeper, 

 I think), the other at the end of the 'neck.' This face of the rock 

 looked a little south of east. 



Since my return to England I have ascertained that an account of 

 these curious hollows was published in 1882 by Prof. H. H. Reusch 

 (Bull. Soc. Geol. France, ser. iii, t. xi, p. 53).^ He states that they 

 occur in both granite and schists, the smaller being called tafoni, 

 the larger grottes, and thinks they indicate soft places in the mass, 

 thus being the converse of the weathered out blocks which simulate 

 erratics. He remarks that he has seen some approach to the structure 

 in the old sea caves of Norway. They are also mentioned, with 

 other cases of peculiar erosion, in Professor Penck's " Morphologie 

 der Erdoberflache " (1891, pt. i, pp. 214-216), and by him attributed 

 to local decomposition, but these in several cases appear to be 

 only basins of the ' sacrificial ' type. Siguor Paul Choflfat, how- 

 ever, gives an excellent description of some examples in Portugal 

 (" Commuuica96es da Direc9ao dos Ti-abalhos Geologicos de Portugal," 

 t. iii, p. 17) with four good illustrations. They occur in a porphyritic 

 granite at two localities very different in situation. One, in the 

 Gerez, is nearly 15 leagues inland on barren hills, almost 3,000 feet 

 above sea-level ; the other, at Faro d'Anha, at heights of from 500 

 to 550 feet in a thick wood of tall pines. There are three types : 

 simple basins, basins with a linear arrangement and more or less in 

 communication by destruction of the intervening walls, and, thirdly 

 (only at Faro d'Anha), varied forms, such as I have described above, 

 one of them being undei'cut, though to a less extent than that 

 figured by Mr. Tuckett : the sides, not in this case only, exhibit 

 a slight, rudely horizontal ridging, with an approach to a glaze. 

 Signer ChofPat attributes these hollows to local decomposition, though, 

 if I rightly understand him, he would not exclude the possibility 

 of some having been ' touched up ' by pi'ehistoric man. He states 

 that the basins (of the Dartmoor type) are not unfrequently sur- 

 rounded by a Slight rim, as if the adjacent rock were slightly harder 

 than the rest, mentioning a suggestion by Professor Heim that this 

 might be due to percolation of silica liberated by decomposition 

 of the felspar — an ingenious idea, worth careful consideration, for 

 it would also explain the occasional approach to a glazing of the 

 surface, but not quite free from difficulty, for this percolation would 

 have sometimes to extend almost against gravitation, and one would 

 expect the hardening of the silica to check the enlargement of the 

 cavity. 



Decomposition seems the natural explanation, but in some of the 

 cases described this must have acted in a singularly selective 

 way ; the granite also, as I have said, shows no sign of anything 



1 I was set on the track of this and other references by Sir A. Geilue's valuable 

 "Text Book" (p. 456). Professor Eeusch also published in a Christiauia scientific 

 periodical (1878), but I have not been able to consult the paper. 



