we 
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“172 Association of American Geologists. te 
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' Dr. James Rogers thpashe, we must consider dolomite as a 
true double salt : 1 atom carbonate of lime + 1 atom carbonate of 
magnesia; the excess of magnesia found in our limestones must 
be considered a mechanical mixture. 
communication was received from Peter A. Browne, Esq, 
expressing a willingness to read before the Association ‘an Essay 
on Aérolites or Meier mani at the next, , annual session. 
Laid upon the table. i 
c Prof. Mather ea verbal ‘communication on the joints in 
as rocks, particularly as y occur in the primar , transition and sec- 
+, ondary of this co He found two principal sets of joints 
"prevail 5 the first had a general direction of north by northeast, 
‘perp endicular to the formers-besides 
e not so smooth andawell mar ed as in oie 
obse ‘vation was not intended to apply to the 
te. 
= Dr. Ja St k titea the joitthe or Scaiene of the fe tem 
around iaasiies and particularly at Roxbury, Mass., and also: in 
‘the island of Rhode Island, at arias called Purgatory, the large 
_ pebbles are broken by these fractures ae t dislocation or loos+ 
% ening” from their beds. He supp the parallel and uniform 
e lime rocks and slates + that country to have con- 
ion with the different epochs of irruption of the trap, aa 
Dr. Douglass Houghton inquired of Dr. Jackson if these 
—- the conglomerate had reference to the line of bearing, 
ee that in Michigalhthey were nearly at — angles to 
Purgatory the pe ebbles were ve ares ovate, cameo! with 
their longer diameters 9 re ( 
- concre bvichbles of quartz into a firm mass—this ae elu- 
_— eidate present subject. Specular iron was generally observ- 
able among the interstices of the. pebbles at Purgatory, and more 
or less of iron and lead ore was generally to found at the junc- 
ture ang Soe of the trap ie 
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