J. M. Clements — Study of Contact Metamorphism. 85 



The relative proportion of the alkaline earths, lime and 

 magnesia, is also striking, the latter being present in the 

 greater quantity. As a general rule, in igneous rocks (and to 

 igneous rocks all clay-slates owe their ultimate origin) the 

 reverse condition exists, i. e., the magnesia is subordinate in 

 quantity to the lime, except in the ultra basic non-feldspathic 

 kinds. The carbon present in this slate is considered as offer- 

 ing trustworthy evidence of the presence of organic life at the 

 time of the deposit of the slates, though no more satisfactory 

 evidence of the existence of life thus early has ever been 

 found. It will be noticed finally that considerable water is 

 present, but in consideration of the character of the rock this 

 is to be expected, and if anything the value is low. These 

 clay-slates are the rocks which are nearest the original sedi- 

 ments. 



Phyllites. 



The phyllites have a silky luster and bluish black color. 

 They are composed essentially of white mica, which occurs in 

 large quantity surrounding grains of quartz and feldspar (?). 

 Rutile occurs in single crystals and in clumps of small crystals 

 scattered through the mica. Associated with it, there occur 

 here and there flakes of hematite, and small aggregates of 

 black, undeterminable specks. Apparently, no interstitial 

 material, such as occurs in the clay slates, is here present. 



These rocks seem to differ from the clay slates only in that 

 they are more completely crystalline, the interstitial material of 

 the slates having disappeared. Owing to the apparently unim- 

 portant differences which exist between the clay-slates and the 

 phyllites, no analyses have been obtained of the latter. 



Spilosites, Desmosites, and Adinoles. 



With these rocks we begin the consideration of the true 

 contact products of the dolerite. These contact rocks possess 

 certain characters in common. They are dense, flinty, " horn- 

 stone-like" rocks, which in some cases still show the fine 

 banding of the original slates. Others are very characteristic- 

 ally spotted. They have a splintery, and at times almost con- 

 choidal fracture, and vary in color on fresh fracture from light 

 to very dark gray and greenish. The weathered surface, in 

 almost all cases, is covered by a thin, white to light yellowish 

 crust. The mineralogical components are quartz, feldspar 

 (albite), biotite, chlorite, white mica, actinolite, rutile, epidote, 



