368 On some Reactions of the Salts of Lime and Magnesia, 
49, Becides the magnesian limestones of parm strata 
a preoee of dolomite occur in the rocks of every geological 
ve long since described the dolomites which form 
posers beds s, often associated with ophiolites and with erys- 
talline limestones, in the Laurentian system in Canada. Great 
portions of the paleeozoic limestones of North America are mag- 
nesian, especially in the valley of the Mississippi,* while deposits 
of dolomites are found in Europe alike in the Permian, Triassic, 
Jurassic, and Tertiary strata. Mr. Dana has even described as 
of recent formation a dolomite from the coral island of Matea, 
beet by Silliman and myself.—(Am. Journal of Science, (2, 
xix, 
50. The — conditions of these magnesian limestones 
vary greatly; they are sometimes made up of crystalline ame 
of dolomite, which are strongly coherent, or more rarely fo 
loose sand. Not unfrequently the magnesian limestones are ea 
cretionary in their structure, and may be oolitic or botryoidal. 
The action of the concreting force has sometimes obliterated the 
marks of stratification. The porous or cavernous structure of 
many dolomites is also to be remarked. 
* For the following facts with regard to the dolomites of the palzozoic rocks of 
the Mississippi valley, I am indebted to Prof, James Hall of A ‘Geer y. We have 
ge gehen same order: 
e so-called Magnesian limestone, which is regarded as the equiva- 
leat o of ‘the Calciferous Sandrock, and is from 200 to 250 feet in thickness. — Kis 
2d. The Galena li ihre baseman ves about 250 feet of dolomite “ntorpoced 
betmeen the Trenton and the Hudson River groups. It is the lead-bearing rock of 
Iowa, Wisconsin 
and Illinois, 
3d. The Niagara li ns sien. doll Sg, 250 Sant in. iene, 20 2 ti 
4th. The Leclaire or Galt 2 rset one, a dolomitic formation interposed between 
the last and the Onondaga Salt Group, It attains upon the Mississippi a thickness 
af 500 foot, but thine out, to the « astward. : 
5th. The fea of the Onondaga Gated 100 feet thick. 
6th. A dolomitic deposit in the rei’ rt of the Carbonifé a rous series. 
The formation No. 1, al lly regarde as the equivalent of the Calcif 
is sienlatiee of the e, which on Lake 
ktatecd sine tien a pure dolomite, and on the island of Montreal includes thin 
sandrock itself, er Canada, eee 
extensive bois of dolomite, and the Hudson udson River group is characterized by beds . 
of dolomite and of magnesite 
