BiNNS. — On a Striated Bock-surface. 335 



bright colours with the crossed nicols ; these about equal 

 the quartz in quantity, sometimes the one, sometimes 

 the other predominating ; very probably they are fragments 

 of a decomposed felspar, but some may be bits of an argil- 

 lite : (3) fragments of well - preserved felspar : (4) some 

 fragments of a rather fibrous brown dichroic mineral, pro- 

 bably biotite : (5) two or three fragments of a filmy green 

 mineral, probably an altered form of (4) : (6) a white mica. 

 The microscopic aspect of the rock would suggest that it 

 was a quartzite ; but the microscope shows no marked 

 deposition of secondary quartz, or any indication of victa- 

 nnorpliism, so that it must be named an indurated, rather 

 felspathic grit. It is not likely to be an Archa3an rock, but 

 has probably derived its materials from rocks of that age, 

 being itself very possibly Palteozoic. There is no very defi- 

 nite indication of either a laminated or a cleaved structure. 



Aet. XL. — On a Striated Bock-surface from Boatmaiis, 



near Beefton. 



By G. J. BiNNS, F.G.S. 



[Read before the Otago Instikde, lltli September, 1S8S.] 



Plate XXV. 



There is no source of geological action more frequently ap- 

 pealed to for elucidation of problems in dynamical geology 

 than ice ; and the various traces of its former presence, in the 

 form of striated boulders and rock-surfaces, roclies viontonnccs 

 and blocs iierches, are familiar to all, if not from actual experi- 

 ence, at least from the text-books. 



It is very necessary, in ascribing great geological results to 

 this agency, to be certain that the evidence upon which we 

 base our calculations — frequently erecting a vast superstructure 

 of hypothesis upon a very small foundation of evidence— is in- 

 disputably true. 



As an example of what might, were its origin not known, 

 have led to misconception and possible error, I beg to bring 

 before this Institute an example of a striated rock-surface, in 

 the grooving of which ice-action had no part. (See Plate XXV.) 

 The fragment of rock forming the subject of this paper was re- 

 moved by me, on the 4th of May, 1888, from the surface of 

 a large mass of debris resulting from a landslip which occurred 

 at Boatman's, near Eeefton, during the preceding March. It 

 consists of a piece of indurated arenaceous clay, containing 

 some mica and indistinct carbonaceous impressions, and its 



