ON THE OBERMITTWEIDA CONGLOMEBATE. 25 



4. On the Obermittweida Conglomerate, its Composition and 

 Alteration. Ey T. G. Bonney, D.Sc, LL.D., P.E.S., V.P.G.S., 

 Professor of Geology in University College, London, and Fellow 

 of St. John's College, Cambridge. (Eead November 9, 1887.) 



The series of specimens from Obermittweida entrusted to me for 

 examination by Professor Hughes is a remarkably fine one. All 

 those of the conglomerate are of a fair size, some weighing several 

 pounds, and are carefully selected so as to show the nature both of 

 the matrix and the included pebbles ; there is also a piece of the 

 neighbouring gneiss. The pebbles are, in form, both well rounded 

 and subangular. Some of the smaller fragments, indeed, occasionally 

 seem to be little, if at all, worn, while the larger, in three specimens 

 at least, are true pebbles. Two of them are full three inches in diam- 

 eter. More than one kind of rock is present. The matrix also varies 

 from a rather coarse-grained micaceous grit, to a hard tolerably uni- 

 form rock, in which the individual constituents can barely be detected 

 by the unaided eye. It is of a purplish-brown colour, and has evi- 

 dently been subjected to a pressure definite in direction, which has 

 produced, especially around the included pebbles, an incipient clea- 

 vage. These divisional surfaces are coated with a filmy mica, pro- 

 ducing what I have already termed " sheen surfaces." As is usual, 

 they bend round the pebbles, like the husk around a nut, so that the 

 presence of a concealed pebble is often indicated by a " varnished " 

 ovoid swelling on the surface of the mass. The matrix, though clearly 

 fragmental in origin, suggests that a certain amount of metamor- 

 phism in situ has taken place, and in this respect it reminds one of 

 that of the Huronian conglomerate, which I have examined at 

 Sudbury (Canada). 



The gneiss has a superficial resemblance to this matrix, but is 

 rather more distinctly micaceous. Poliation is not very strongly 

 marked ; the specimen brought exhibits neither a distinct mineral 

 banding nor a marked fissility. 



In my study of these rocks I have not attempted to identify with 

 precision every micro] ith either in matrix or pebbles. My purpose 

 has been to investigate, as far as possible, the history of the rock, 

 and to see what light it throws on the subject of metamorphism *. 



As it happens, my own collection, both of fragmental rocks of 

 considerable geological antiquity and of schists, gneisses, and other 

 rocks usually called metamorphic, is a rather large one ; and to the 

 examination of these and of specimens kindly lent by friends I have 

 of late years devoted much time. I have also a fair series of rocks 

 in which the changes have been mainly due, in the one case to 



* It is the less needed because the rock has been the subject of an elaborate 

 study by Dr. A. Sauer (Zeitschr. gesammt, Naturwissenschaften, Halle, 1877), to 

 whose courtesy I am indebted for a separate copy of his paper. The rock also is 

 noticed by Dr. J. Lehmann, ' Enstehung der altkryst. Schiefergest,' ch. vii. 



