134 DYNAMICAL GEOLOGY. 



The change, if produced through the magnesium chloride (MgCl), required tlie removal 

 of ^ Ca by the chlorine of an equivalent amount of Mg. If this is the true theory of dolo- 

 myte-making, then great shallow areas or basins of salt-pan character must have existed 

 in past time over various parts of the continental area and have been a result of the oscilla- 

 tions of the water level. Such magnesian limestones contain few fossils, partly because of 

 the fine trituration, and partly, no doubt, because of the unusually briny condition of the 

 waters. The frequent alternation of calcite and dolomyte strata would indicate alternations 

 between the clear- water and salt-pan conditions. Dolomization, in the case of such beds, 

 has often taken place after partial or complete consolidation ; for many dolomytes are 

 exceedingly porous, because of the diminished bulk of the dolomyte — one eighth to one 

 tenth. T. S. Hunt made the porosity of several Canadian Lower Silurian dolomytes, 10 

 to 13^ percent (1866). 



Local cases of alteration are well known. Adolf Schmidt mentions such at the lead 

 mines of Missouri, which he attributes (following Bischof) to the action of magnesium 

 bicarbonate. 



In a memoir on the famous dolomyte region of the Tyrol, Dolter and Homes, geol- 

 ogists of Vienna, discuss this subject at length, and reach the following conclusions : 

 (1) Some large limestones, weakly dolomitic, may have been made out of those organic 

 secretions which contain a little magnesia ; (2) minor cases of the production of dolomyte 

 are due to the alteration of limestone through the introduction of magnesium carbonate ; 

 but (3) the larger part of dolomyte formations, whether more or less rich in magnesia, have 

 been formed from organic calcareous secretions through the action of the magnesium salts 

 of sea water, especially the chloride. 



(d) Making of day and soil. — Pure white clay, or kaolin, used in mak- 

 ing porcelain, is sometimes in strata of wide extent ; and the common impure 

 river-valley clays, employed in brick-making and coarser pottery, have no 

 less value. One of the largest kaolin beds in New England, at ISTew Marl- 

 boro, in Berkshire County, Mass., was probably made by the decomposition of 

 the orthoclase that was disseminated through quartzyte, and its removal by 

 percolating waters to the bed of a streamlet ; for in other localities in 

 Berkshire this result is now going on from the same quartzyte. The absence 

 of black mica and other iron-bearing minerals insured its being white. 



(e) The blanching of red and rusty rocks by waters containing carbonic acid 

 and organic acids or materials is a common and important effect. Colored 

 clays are drained of their iron oxide and whitened by percolating waters. A 

 deeply rusted block of basalt or granite may thus be made to have a white 

 exterior an inch or more deep. 



(/) Again, the impurities of a limestone are sometimes raade available for 

 soil, by the continued action of carbonated waters, and the removal thereby of 

 the calcareous part. Shells and corals contain about 0*5 per cent of impurity, 

 consisting chiefly of iron oxide and alumina; and the action of the rains over 

 the hills of coral sand-rock on Bermuda, through centuries past, has left a 

 residuum of red earth which is the soil of the island, as Wyville Thomson 

 suggested. The red ooze or mud over much of the ocean's bottom below 

 2500 fathoms is due chiefly to the removal, in like manner, of the calcium car- 

 bonate of the Globigerinae and other Rhizopods, in consequence of an excess 

 of carbonic acid in the bottom or abyssal waters. The life of the sea-bottom 



