25 l I r an Twyl — Neio Points "it the Origin of Dolomite. 



irregular patches of dolomite on the borders of dolomite 

 masses ; (3) the existence of remnants of unaltered limestone in 

 dolomite, and of nests of dolomite in limestone; (4) the irregu- 

 lar boundaries between certain beds of limestone and dolo- 

 mite; (5) the presence of altered oolites or fossils in many 

 dolomites; (6) the protective effect of shale beds ; and (7) the 

 obliteration of structures and textures. 



In some instances the relationship of dolomite to limestone 

 is such as to indicate that the alteration was accomplished by 

 solutions which migrated from above down wards after the lime- 

 stone was formed, or at least in the closing stages of its forma- 

 tion. ■ 



It is an interesting fact that certain layers have sometimes 

 been passed over during the dolomitization of adjacent ones, 

 and show little or no sign of alteration. The so-called inter- 

 stratification of limestone and dolomite cited by some as evi- 

 dence in favor of some primary theory of origin is then, in 

 some cases at least, rather a pseudo-inter-stratification produced 

 by the selective dolomitization of an original limestone. Some 

 layers which have been passed over have been noted to be 

 coarser grained than the adjacent layers which have been 

 altered and this would seem to explain their greater resisting 

 power. At times, however, the unaltered layers do not appear 

 to differ markedly from the altered ones. The phenomenon is 

 then difficult to account for. Normally the contact lines 

 between such interbedded layers of limestone and dolomite are 

 fairly regular and definite, but in some instances they are 

 known to be very irregular and may even simulate irregular 

 contacts produced by disconfortnity. A remarkable example 

 of a pseudo-disconformity produced by uneven selective dolo- 

 mitization has been observed in the St. Louis limestone near 

 Farmington, Iowa. Here a bed of altered limestone is found 

 resting very irregularly on a bed of dolomite. The two beds 

 are very different physically and might readily betaken at first 

 sight for two distinct formations, but when the contact is 

 traced laterally for a short distance the lower bed loses its dolo- 

 mitic character and passes into a limestone very similar to and 

 continuous with the bed above. 



Another striking relationship of limestone to dolomite is 

 exhibited in a certain layer of an interbedded series of lime- 

 stones and dolomites of the Beekmantown in the old Walton 

 Quarry near Harrisburg, Pa. The beds dip south here at an 

 angle of 30°. The layer in question is represented by dolo- 

 mite six feet in thickness in the upper part of the quarry face 

 and on each side of it appear good limestone layers. Now in 

 the lower part of the quarry the lower half of this layer passes 

 abruptly into limestone and continues to the quarry floor as 



