W. S. Bayley — Spotted Rocks from Minnesota. 391 



The rocks immediately associated with the spotted rocks 

 in the same beds, but which themselves are free from spots, 

 differ but slightly from the last described rock in structure. 

 They contain, however, quite a large quantity of chlorite, 

 which has clearly been derived from biotite on the one hand, 

 and from feldspar on the other. Closely intermingled with 

 this, but more particularly in the neighborhood of little calcite 

 nests numerous irregularly shaped plates of a light green, 

 strongly pleochroic and brightly polarizing epidote are fre- 

 quent. In most cases however the epidote occurs in tiny 

 rounded grains and rudely outlined crystals scattered every- 

 where throughout the feldspathic and chloritic groundmass in 

 which the enlarged quartzes are imbedded. These little parti- 

 cles are sometimes crossed by cleavage cracks parallel to which 

 extinction takes place. They have a faint greenish tinge and 

 a very high refractive index. In addition to chlorite and epi- 

 dote the rocks of this class sometimes contain flakes of brown 

 biotite, magnetite and calcite.- 



The groundmass of the spotted rocks when examined under 

 the microscope, presents the same features as are observed in 

 the epidote-bearing rocks just described, except that the epi- 

 dote grains are much less abundant and in some cases are 

 almost entirely lacking. 



The spots, on the contrary, are usually very rich in epidote. 

 They consist of enlarged quartz grains, a little feldspar and 

 occasionally a 'small quantity of chlorite, all imbedded in a 

 mass of calcite and epidote. The amount of calcite present 

 varies within wide limits. It sometimes occupies nearly the 

 entire space between the quartz, feldspar and chlorite to the 

 almost complete exclusion of the epidote. In other cases it 

 occurs only sparingly, while the epidote is massed in little 

 plates, grains and crystals. Only rarely were well terminated 

 crystals observed ("No. L 1.463). These average ■£■% of a milli- 

 meter in length. A few forms are represented in figure 2. 

 Although but very slightly pleochroic, 

 the true nature of the crystals cannot 

 be doubted. In color they are of such 

 a very pale green tint as to be almost 

 colorless. They possess a cleavage 

 and an extinction parallel to their 

 long axes, have a high inde*x of re- 

 fraction and are usually free from 

 inclusions. They are distinguished 

 from sahlite, which they closely re- 

 semble, by the fact that the plane of 

 their Optical axes is at right angles to Flg - 2 ' x 200- 



their cleavage. 



