4 g2 G. L. VOSE ON THE DISTORTION 



XII. On the Distortion of Pebbles in Conglomerates, with Illustrations from Rangelg Lalce, in 



Maine. By Geo. L. Vose. 



Read January 3, 1868. 



JL HE attention of geologists, both in Europe and in this country, has recently been directed 

 to certain changes which seem to have taken place in masses of conglomerate, by which the 

 included pebbles appear to have been altered both in outward form and in their mineral 

 character. The late Dr. Edward Hitchcock, and his son, Mr. Charles H. Hitchcock, have 

 called attention in several different publications, since the year 1860, to localities in Maine, 

 Vermont, Massachusetts, and Rhode Island, where this metamorphism is said to have oc- 

 curred. The above-named geologists maintain : First, That the pebbles of certain conglom- 

 erates have been elongated, flattened, and bent, since their consolidation into rock, i. e., since 

 they were pebbles, — and also, that the mineral nature of some of the pebbles has been ma- 

 terially changed during the process of compression, to which the flattening is believed to be 

 due. Secondly, That the operation has, in some cases, been carried to such an extent that 

 the pebbles have been entirely flattened out, and changed into the siliceous lamina? of tal- 

 cose and micaceous schists, while the cement has been converted into mica, talc, and feldspar. 

 Finally, The extreme result of this process of metamorphism is stated to be granite, and 

 instances are adduced where the various changes may be traced in unbroken connection — 

 a single undivided ledge passing from an undoubted conglomerate to a rock, which by itself 

 would be called granite ; so that, as the Messrs. Hitchcock observe, the conclusion is forced 

 upon us that the granite was formed out of the conglomerate. The principal agent in this 

 operation is considered to have been severe compression, acting at right angles to the plane 

 of the dip and strike ; and the materials are supposed to have been at the time in a plastic 

 condition. 



With regard to the conglomerate referred to by the above-named geologists, at Purgatory, 

 near Newport, Rhode Island, it was held by Dr. Charles T. Jackson and Prof. Wm. B. Rogers, 

 and by Mr. B. S. Lyman of Philadelphia, that the arrangement of the longer axes of the peb- 

 bles into parallel planes is such a one as they would naturally assume from the action of 

 waves upon a beach ; that the flattened form of the pebbles was due to the schistose struct- 

 ure of the rock out of which they were formed ; and that even an apparent bending of one 

 pebble around another might result from the accidental juxtaposition of a round pebble 

 and a pebble formed from a part of a sharply folded bed of schist. It was also held by these 

 gentlemen to be very improbable that the pebbles had been in a plastic condition since 

 their formation. 



The distortion of the pebbles in the Newport conglomerate, is by no means extreme, and 

 this locality alone might not be sufficient to place the conclusion, that the pebbles had been 

 altered in form, beyond doubt ; but the locality to which we are about to refer, seems to 

 leave no room whatever for opiestion as to the extreme change which the pebbles have 

 undergone, both in shape and mineral character ; and we believe that the nature of the 

 agent which produced the alteration, and the condition of the materials at the time they 

 were altered, may both be understood with a considerable degree of certainty. 



Mr. Charles Hitchcock, in his reports upon the geology of Maine, refers to the conglom- 

 erate at the east end of Rangely Lake, in the upper part of Franklin County, as affording 



