proceedings: geological society 295 



phenocrysts are speroidal or elliptical and as much as three inches long 

 by two inches in diameter. They weather out and look like rounded 

 pebbles. The main intrusion is exposed for about three miles along 

 the creek but it is not known at what angle the creek cuts the dike. 

 Smaller dikes are from fifty to several hundred feet broad and all are 

 crushed to gneisses along their sides. 



D. E. Winchester: Contorted hittiminous shale of Green River 

 formation in Northwestern Colorado. In the upper part of the oil shale 

 series of northwestern Colorado there is a zone of rich black bituminous 

 shale which is everywhere contorted showing on its weathered surface 

 minute folding and faulting. This shale which has been observed 

 over a wide area is overlain and underlain by beds of laminated and 

 uncontorted shale and sandstone. The bituminous shale itself con- 

 tains a large amount of microscopic vegetable material, including algae, 

 etc., and this may furnish a clue as to the reason for its contorted con- 

 dition. The region is one in which there is practically no faulting, and 

 beds dipping as much as 15° are the exception rather than the rule. 

 It is concluded that the twisting of the laminae occurred before the 

 bituminous shale was completely solidified and that the beds containing 

 the great amount of vegetable matter were least competent and may 

 therefore have taken up any movements which may have occurred in 

 the region. vSpecimens of thinly laminated shale rich in organic matter 

 but occurring at a horizon 300-500 feet below the black contorted beds, 

 show intricate folding and minute faulting. 



Discussion; E. O. Ulrich: The same contorted condition of thin beds 

 can be seen in mud laid down on tidal flats and then slumped by tidal 

 undercutting. It is shown also in West Canada Creek at Trenton 

 Falls, New York, where the limestone, once a limy mud, has these fea- 

 tures. C. D. White: Green River beds essentially horizontal are a 

 good place to see thin contorted strata. The contorted beds are buried 

 beneath shale and sandstone. It would seem that the organic beds 

 between sandstones must have been slimy mud a long time after burial 

 and even if somewhat hardened must have been the easiest zone for 

 slipping. They doubtless were somewhat solidified when the crumbling 

 took place else there would be no faulting. Possibly the shale exhibited 

 by Winchester was deeply buried and the contortion is due to movement 

 along the oil shales after they were partly solidified. The overlying 

 and underlying formation,s have a somewhat reinforced structure and 

 slight jars or lesser earth movements would be taken up by oil shale 

 beds which are rich in vegetal matter. Similar structure is found in the 

 Elkhorn coal on Marrowbone Creek in eastern Kentucky, where the 

 upper part of the coal bed moved on the lower part. W. C. Alden: 

 This kind of structure is common in glacial lake beds where we find 

 zones a few inches thick highly contorted, while above and below are 

 beds clearly and beautifully laminated, which show no disturbance. 

 It is suggested that the contortion may be due to freezing before the 

 superincumbent layers were deposited. G. H. Ashley: In northern 

 Indiana the plankton found in the small lakes resembles in structure the 



