42G 



SCIENCE. 



[X. S. Vol. XXI. Xo. 533. 



On Zirkon from Lawton, Oklahoma: George 

 F. KuNz. 



On Monazite Sand from Idaho : George F. 



A paper entitled ' The Serpentine and Asso- 

 ciated Asbestus Minerals of Belvidere Moun- 

 tain, Vermont,' was presented by V. F. Mar- 

 sters, of Columbia University. 



Belvidere Mountain lies approximately along 

 the line between the counties of Orleans, 

 Lamoille and Franklin. It is a sharp crested 

 ridge with a maximum elevation of some 

 2,100 feet above Eden Corners at its southern 

 termination. Three topographic elements are 

 prominent, a sharp crested ridge forming the 

 upper 900 feet of the mountain, a cresoentic 

 plateau with a flat top 1,200 feet above the 

 valley floor and rimming the end of the moun- 

 tain, and lastly a steep lower slope composing 

 the foot of the plateau and extending to the 

 valley bottom. 



The upper part with steep slopes is composed 

 of amphibolite. In addition to the hornblende 

 which makes up seventy-five per cent, of the 

 rock, there is also present an inconsiderable 

 amount of epidote and a non-pleochroic color- 

 less mineral regarded as zoisite, together with 

 magnetite and pyrite. Towards the base, 

 garnet becomes a prominent constituent, suffi- 

 cient to make a well-defined garnet zone. In 

 nearly all cases observed, the garnet is largely 

 altered to penninite, a variety of chlorite. 

 Along the garnet zone the hornblende has also 

 undergone marked alteration in part to serpen- 

 tine. The nose-like projection forming the 

 plateau is composed of serpentine. In this 

 rock occur the so-called asbestus deposits re- 

 cently prospected and worked for this product. 

 In thin sections the serpentine appears to be 

 made up largely of a felty and fibrous mass, 

 apparent only under cross nicols. It is typical 

 fibrous serpentine. In thin sections from the 

 upper part of the plateau and in close prox- 

 imity to the overlying amphibolite, there ap- 

 pear shredded masses presenting the original 

 structure of hornblende as seen in the am- 

 phibolite, but mineralogically altered to a 

 fibrous mass with the optical characteristics of 

 anthojjhyllite. It is not improbable, moreover, 

 that a portion of the hornl)]ende has altered to 



tremolite. These fibrous con,~tituents form 

 the so-called ' slip-fiber.' 



The serpentine belt has also betn subjected 

 to peculiar faulting and crushing. The cracks 

 thus produced, even on a microscopic scale, 

 have been filled with these fibrous constituents, 

 and then the whole mass submitted to further 

 slipping. This has caused the slickensiding 

 phenomena on the fracture planes and a con- 

 sequent stretching of the fibrous content; 

 hence the term ' slip-fiber.' ' Cross-fiber ' or 

 true chrysotile is to be found in this area. It 

 is best developed along lines of maximum 

 fracture and minimum lateral thrust. There 

 appears to be two bands of maximum fracture, 

 one extending along the upper portion of the 

 plateau and not far from the garnet zone, the 

 second along the foot of the plateau and best 

 shown on the property of Judge Tucker. 



The next paper was by Dr. Charles P. 

 Berkey, on the ' Interpretation of Certain 

 Laminated Clays with their Bearing upon 

 Estimates of Geologic Time.' 



Laminated clays of glacial and post-glacial 

 age are abundant in many districts of the 

 northern states and Canada. They are espe- 

 cially abundant about the head of Lake 

 Superior where their origin is intimately re- 

 lated to the closing fluctuations and final with- 

 drawal of the Wisconsin ice sheet. 



One of these deposits, at Grantsburg, Wis., 

 exhibits a remarkable uniformity of structure 

 and is so clearly bounded by other accumula- 

 tions of known significance that its history is 

 readily interpreted. From a detailed analysis 

 of the laminated structure it is argued that 

 this deposit was about 1,700 years in accu- 

 mulating. 



A like interpretation of the similar isolated 

 deposits following the retreating ice sheet 

 would give data for time estimates from an 

 entirely new standpoint. In some areas lami- 

 nated clays occupy interglacial position and 

 it may be possible to apply the same method to 

 them. 



1'he last paper of the evening wa> by Pro- 

 fessor A. W. Grabau, on the 'Evolution of Some 

 Devonic Spirifers.' Spirifer mucron<itus (Con- 

 rad) is a Linnffian species comprising a large 

 nunilier of mutations. A remarkable fact ia 



