Association of American Geologists. 173 



Prof. Hitchcock thought that the steps of the new red sand- 

 stone of the Connecticut valley were the result of the fractures 

 referred to by Prof. Mather — they were nearly coincident with 

 the strike of the strata, as if caused by elevatory movements. He 

 found difficulty in distinguishing between fissures produced by 

 mechanical violence and joints properly so called ; he viewed 

 those of the conglomerate as mechanical, those of the slates as 

 chemical. Two cases occurred to him as worthy of notice ; the 

 first was of a dyke of greenstone crossed by parallel transverse 

 planes two or three feet apart and at right angles to the strike of 

 the vein. The second case seemed to throw some light on the 

 origin of this class of phenomena ; it occurred in a bed of the 

 common blue diluvial clay — the horizontal layers were unmoved, 

 but some of them were divided into double rhombs. The exper- 

 iments of Mr. Fox, of England, in the lamination of day by gal- 

 vanism, seem to explain this structure. 



B. Silliman, Jr. had found this rhombic structure in great 

 perfection in the argillaceous sandstone of the Connecticut valley 

 at Hartford, in Connecticut, where this variety of sandstone is 

 used for flagstone. Many of the joints parallel to this rhombic 

 structure are filled with carbonate of lime. 



Dr. C. T. Jackson stated that the great trap dykes of Nova Sco- 

 tia had the perpendicular columnar structure in a high degree — in 

 the smaller dykes this structure prevails from side to side of the 

 dyke, perpendicular to the walls. He thought that in all cases 

 these phenomena were referable to the way in which the dykes 

 cooled — the structure being perpendicular to the cooling surface — 

 thus the narrow dykes cooled from side to side, and the heavy 

 ones from the upper surface downward. 



Prof. Henry D. Rogers remarked, that the trap dykes of 

 Pennsylvania and the magnetic iron ore of New Jersey were 

 abundantly characterized by the columnal structure. He viewed 

 the horizontal columnar structure of the magnetic iron ore, as a 

 very important indication of its igneous origin. 



This discussion was here suspended to give an opportunity for 

 Prof. Bailey to read his paper on recent and fossil infusorias.* 



* As this paper of Prof. Bailey is to appear in full, with plates, in the October 

 No. of this Journal, it has been thought unnecessary to attempt here any abstract 

 of his remarks before the Association. — B. S. Jr. 



