54 MR. J. W. GREGORY ON THE 



A dense fir-forest, however, clothes the whole slope and prevents 

 any very satisfactory evidence being obtained. Nevertheless, here 

 and there a knob of rock rises above the soil of pine-needles, the 

 path beside the stream shows an occasional low section, and the bed 

 of the stream affords a little additional information as to the general 

 nature of the diabase and its varieties. The principal types are : 

 (1) a compact non- vesicular diabase ; (2) a weathered amygdaloidal 

 diabase; (3) the variolitic diabase, and this is usually associated 

 with spheroidal varieties. 



The compact diabase varies considerably in coarseness : some is 

 quite microcrystalline, some is full of delicate acicular crystals of 

 felspar ; while iu other places it is very coarse, as at about 80 yards 

 on the Berneck side of the drinking-fountain. One of the most in- 

 teresting varieties of this group of diabases may be seen crossing 

 the Metzlersreutherbach at the south end of the meadow which 

 stretches from the Oelschnitz up the course of its tributary. This 

 rock stands out from the slopes on either side as a dyke-like wall that 

 forces the stream into a small cascade. The rock is much altered ; 

 zoisite has been extensively developed in the plagioclase, and the 

 whole of the pyroxene has been converted into a dichroic, chloritic, 

 greenish mass; the most marked feature in this — as in most of 

 these compact diabases — is the great abundance of leucoxene, result- 

 ing from the alteration of ilmenite. Though the specific gravity of 

 this rock is high, and it probably originally contained an excess of 

 iron in the form of primary ilmenite, no doubt most of this was 

 secondary, and produced as a result of the decomposition of the 

 augite. Specimens can be collected which are quite white from the 

 great abundance of the leucoxene. 



The compact diabases are, however, rather exceptional, and the 

 weathered amygdaloidal varieties occupy the largest part of the area. 

 The commonest type of this is a rock with a bright green, strongly 

 dichroic matrix, crowded with actiuolite needles and grains of 

 leucoxene ; some of the latter still retain part of the original 

 ilmenite. The vesicles are mainly filled with calcite, but some 

 zeolites are present. 



The sections are so scrappy that the relations of the two classes 

 of diabase cannot be accurately determined. It is possible that 

 some of the compact diabase may be in the form of dykes intrusive 

 into the amygdaloid, but in several cases a gradual passage 

 between the two can certainly be traced. The relations of the 

 normal and variolitic diabases to one another, and of both to the 

 Devonians, is, however, clearly shown at several places along the 

 Miihlleite. Two large and several small bands of the Devonians 

 can be seen in the bed of the Oelschnitz and traced up the banks 

 on each side ; tongues of the diabase may be seen intrusive into 

 these. One of the best cases is a short section cut by the lower 

 path, in which a bed of lydianstone, a baked shale, is irregularly 

 overlain by the amyj^daloidal diabase, and in this is a band of still 

 more altered De\onian. Microscopic examination of this rock 

 shows that it was once a shale with a few layers of grit, and that it 



