proceedings: geological society 407 



made by the speaker reduced the estimated rate of recession from the 

 former figure of 4.2 feet a year to 4 feet. This rate would increase the 

 age of the Falls to 41,000 years. 



David White: The occurrence of transported hoivlders in coal beds. 

 The discovery of stray bowlders in the midst of coal seams is a rare 

 event, but often where one is discovered others are found in the same 

 locality. Several occurrences in the Appalachian field were mentioned, 

 in Tennessee, Pennsylvania, etc. A late discovery in the New River 

 coal field of West Virginia has been reported to the Survey, and a speci- 

 men from this find was exhibited. The bowlder was of quartzite, well 

 rounded, and lacquered with coal. The speaker suggested that the 

 character of the rounding seemed to indicate fluviatile action. In ex- 

 planation of the occurrence of the bowlders it was considered probable 

 that a more or less open lane through the coal swamp had existed, per- 

 mitting the transportation of bowlders by floating drift. 



REGULAR PROGRAM 



B. S. Butler: Relation of ore deposits to different types of intrusive 

 bodies in Utah. The larger intrusive bodies of Utah are of two types, 

 laccoliths and stocks. The laccoliths occur in the sandy and shaly 

 sediments in the southeastern part of the state; the stocks in the 

 quartzites and limestones in the western part of the state. The stocks 

 may be subdivided into those truncated near the apex and those trun- 

 cated at greater depth. The deeper truncated stocks are uniformly 

 more siliceous. The apically-truncated stocks are monzonitic to dior- 

 ritic in composition, the deeper truncated stocks have the composition 

 of granodiorite to granite. The ore deposits associated with the lacco- 

 liths and deeper truncated stocks have been of comparatively slight 

 commercial importance, while associated with the apically-truncated 

 stocks are deposits of great value. It is believed that the lack of 

 large deposits associated with the laccoliths is due to the fact that after 

 intrusion they were sealed oft' from their deep-seated source and that 

 the amount of material in the laccoliths themselves was too small and 

 the differentiation on solidifying too incomplete to furnish large de- 

 posits. It is believed that in the stocks the differentiation was greater 

 at depth and that the mobile constituents of the magma, as the water 

 and other mineralizers, with silica, metals, sulphur, etc., in solution, 

 rose toward the surface, while the heavier minerals that crystallized 

 early sunk to greater depth. When the mobile constituents reached 

 a point where the magma was sufficiently solidified to fracture they 

 were guided by the fractures or fissures, and on reaching favorable 

 physical and chemical environments began to deposit the metals in 

 solution. The deeper truncated stocks are regarded as probably rem- 

 nants from which the portion in which the metals were concentrated 

 has been eroded. 



Discussion: Sidney Paige said he was interested in the speaker's 

 views regarding mineralization around laccohths. In the Black Hills 

 the laccoliths had been fractured vertically, and these fractures 



