220 



both in adinole and spilosite. It occurs in short stumpy columns possess- 

 ing very high refraction and double-refraction, and showing the character- 

 istic heart-shaped and knee-shaped twin-forms. It never occurs in the 

 slender acicular form characteristic of the clay-slates, although it may be 

 found in this form (Shenck) in the immediate neighbourhood of the 

 altered rocks. A thin dull white crust containing kaolin is formed by 

 weathering. 



The rock described as adinole from the neighbourhood of Tremadoc 

 differs from the typical adinole of LOSSEN in containing scales of chlorite 

 and mica. It resembles it, however, in general aspect and in mode of 

 weathering. The latter point is one of importance, as it accords with the 

 view that the colourless mosaic is formed in part of felspar. 



The old flagstone quarry of Penmorfa, also near Tremadoc, furnishes 

 other examples of contact metamorphism. A banded structure is very 

 apparent and there is evidence of crumpling in some of the bands. 

 Coarsely spotted rocks also occur. The banded varieties of contact rock 

 were termed desmoisite by ZINCKEN. They resemble the spilosite in com- 

 position. In addition to the constituents already mentioned, the rocks 

 from the flagstone quarry contain patches of calcite which extinguish 

 simultaneously over large areas. Some of the larger colourless grains of 

 the ground-mass can be positively identified as twinned plagioclase, and 

 the extinction angles agree with those of albite. The following analyses by 

 KAYSER show the composition of the contact rocks of the Hartz : 



I. II. III. 



Si0 2 ... 54-02 ... 55-06 ... 73-63 



A1 2 3 ... 21-22 ... . 19-75 ... 15-81 



Fe 2 3 ... 2-51 ... 1-83 



FeO ... 6-48 ... 7-55 ... 0-74 



MnO ... 1-74 



MgO ... 3-01 ... 2-21 ... 1-21 



CaO ... 1-64 ... 3-59 ... 1-02 



Na 2 ... 3-36 ... 7'51 ... 8-33 



K 2 ... 3-71 ... 0-84 ... 0-75 



H 2 ... 1-97 ... 1-83 ... 0-61 



Organic matter tr. ... tr. ... 



99-66 100-17 101-10 



Sp. Gr. ... 2-778 ... 2-813 ... 2-678 



I. Spilosite, Heinrichsburg. II. Desmoisite, Heinrichsburg. III. Adinolefels, Heinrichsburg. (1) 



These analyses show that the distinction between spilosite and des- 

 moisite, on the one hand, and adinole on the other must be due to an 

 original difference in the character of the sediment. A striking feature is 

 the large percentage of soda which the rocks contain. This has been 

 proved by SHENCK w to be due to an actual impregnation of the sediment 



(1) The analyses are taken from LOSSEN'S paper, already quoted. 



(2) Die Diabase dcs oberen Ruhrthals. Inaug. Diss. Bonn, 1884. 



