219 



earthy material of the original sedimentary rock, mixed perhaps 

 with some chlorite. 

 (5) Black, opaque grains without metallic lustre. These are probably 



organic matter. 





(6) Pyrites was observed in one case. 



(7) The spots are formed mainly of chlorite. Sometimes they show a 



colourless border, sometimes a colourless nucleus. Single spots are 

 more or less circular. By the coalescence of several spots oval or 

 irregular forms are produced. 



The existence of an isotropic substance in spilosite is, perhaps, some- 

 what doubtful. Prof. ROSENBUSCH says that he has not been able to 

 detect it in specimens which he has examined. 



The spilosite from the neighbourhood of Tremadoc does not answer 

 precisely to the above description of the Hartz rock. Thus, the chlorite of 

 the mam mass often occurs in well defined scales with definite optic 

 characters. The spots can scarcely be recognized under the microscope 

 without the use of polarized light. Under crossed nicols they are at once 

 defined by their poverty in the vividly polarizing mica. They appear dark. 

 This darkness may be due to the presence of an isotropic substance, but it 

 is more probably due to the presence of extremely minute overlapping 

 chlorite scales. At best, the double refraction of the chlorite is extremely 

 slight, and if scales overlapped compensation might be produced. 



On the slopes of Y-Gesell, also near Tremadoc, another very different 

 type of altered rock may be observed. It is compact, bluish grey in the 

 centre but passing to white at the exposed surface. Tested with a knife 

 it is found to scratch with difficulty, whereas the former rock scratches 

 easily. There are no spots visible. This appears to be the kind of rock 

 which hi other localities is quarried for honestones. It answers to the 

 adinole of the Hartz geologists. Under the microscope the rock is seen to 

 consist of a micro- or crypto-crystalline aggregate of colourless minerals 

 having the refraction and double-refraction of felspar, minute scales of 

 mica and chlorite, and a few ragged specks of a substance which appears 

 opaque by transmitted and white by reflected light (probably leucoxene 

 after ilmenite). The mica and chlorite scales lie in the colourless aggregate. 

 They are far less abundant in this rock than in the spilosite. One or two 

 of the larger grains of the colourless aggregate show twinning, but as a 

 rule the grains are simple, and so small that it is impossible to say whether 

 they are quartz or felspar. 



LOSSEN describes (1) the adinole of the Hartz as a compact, splintery 

 rock, possessing approximately the hardness of quartz. It is generally 

 light grey in colour, more rarely greenish grey. It consists essentially of a 

 fine grained microscopic mosaic of quartz and soda-felspar (albite), the 

 former mineral predominating over the latter. Calcite and pyrite also occur 

 in some cases. ROSENBUSCH (2) calls attention to the existence of rutile 



(1) Erliiuterungen zur geologischen Specialkarte von Preussen. Blatt Har/gerode. Berliu, 

 1882, p. 35. 



(2) Mik. Phy. d. Mass. Gest. 2nd Edit , 1886, p. 238. 



