DETRITAL FELDSPARS IN CAMBRIAN CONGLOMERATES. 211 



enlarged by the growth of a new feldspar, possibly albite, which, with the 

 mica and independent albitic feldspar grains, some secondary quartz and 

 magnetite, are all products of metamorphism, formed in place in the clastic 

 rock. It seems quite clear that the kaolinization preceded the deposition of 

 the conglomerate. 



Now whence came these detrital feldspars ? Surely not from attrition of 

 pebble on pebble. I can imagine no other source than the debris of the 

 deeply decayed mantle. 



In these basal conglomerates occur pebbles of granite and schists, and, in 

 rare instances, of limestone. The relative rarity of rock-pebbles as com- 

 pared Avith those of quartz is due, doubtless, to the fact that, in a disintegra- 

 tion-mantle, cores with unaltered centers are comparatively rare as regards 

 most rocks. Some crystalline rocks are reduced in situ almost wholly to 

 kaolin and quartz in the upper part and to a fine gruess (or decomposition- 

 residuum) in the lower zone. 



The transgression which, in this case, ushered in the Cambrian, found the 

 dry land deeply disintegrated. The breaching waves and currents removed 

 to a distance the kaolin and other fine materials. The lower zone of semi- 

 kaolinized material, and the still lower one in which the feldspar crystals 

 were loosened by the alteration of the micaceous and hornblendic constituents, 

 furnished the fragments of feldspar and the less altered cores of blocks for 

 the conglomerate beds. 



Before applying this hypothesis to the explanation of facts in the Green 

 mountains, I shall give the evidence we have that they were dry land before 

 the Cambrian. 



Evidence of the Stamford Dike. — Clarksburg mountain, a spur of the Green 

 mountain anticlinal near Williamstown, Massachusetts, is an oval mass of 

 granitoid gneiss, mantled around its three sides with the Cambrian quartzite 

 in which Mr. Walcott found Cambrian trilobites. On the eastern flank of 

 the mountain, Mr. Wolff" found a dike of basic rock in the granitoid gneiss, 

 and this dike stopped short at the contact of the gneiss with the quartzite. 



In digging to expose the contact relations, Mr. Whittle and myself found 

 that the dike had been decayed and washed out before the quartzite was de- 

 posited, leaving an open fissure several feet deep and wide, for the beds ot 

 quartzite thicken and sag into the fissure, and contain, at the bottom, material 

 contributed by the decayed dike (figures 1 and 2, page 212). 



We have, in this, evidence of unconformity by erosion and of dry land, 

 as well as of previous rock-decay. 



Evidence from the Green Mountains. — One of the chief difficulties we met 

 with, in studying the rocks of the Green mountains, was the sudden change 

 from true quartzites overlain by the lower Silurian limestone in the valley, to 

 white gneisses overlain by schists on the main ridge east of the valley. 



